Amber is a genre about secrets, but MUSHes are medium for in-character conversation. The value of secrets need to be preserved, yet a secret that is too tightly held means that it might not have the dramatic and play-creating impact that it could have. Moreover, information creates a reason to have conversations, and consequently, having information is extremely valuable; many people are interested in ferreting out information for one reason or another.
Color and Drama
There are two types of information: color, and drama.
Color is exactly that — it is the ornamentation. Color is the little bits of more trivial information, things that decorate the scenes or the lives of characters. It's what color the carpet is, what a character's favorite drink is, and the like. Color is important because it creates the backdrop against which the story takes place, which makes a place and its characters seem real. Discovering information that's color is part of the process of getting to know another character and the world; it functions as a way to connect emotionally to other people and to the setting itself, providing a sense of the environment in which your character lives.
Drama, on the other hand, is information that leads to action. Implicit in the revelation of any dramatic piece of information is the question, "So what are you going to do, now that you know this?" Dramatic information is important because its revelation typically leads to conflict and consequences.
Learning the Secrets of Others
Most secrets are simply learned through conversation — gain someone's trust, and they may reveal dramatic information to you.
However, some secrets are held by NPCs. This involves questions like, "Who was in this tavern last night?" and so forth. There has to be a mechanism for resolving these kinds of conflicts, which we consider akin to any other sort of conflict on the MUSH.
If it's just color information, players should simply negotiate. In general, you should never try to get out of an NPC, information that you could reasonably get IC from a PC. Interact whenever you can. If you want to persistently check up on a particular character, you can let them know that — but it's up to them to come to you for information; don't pester them for it.
Dramatic information, on the other hand, is ultimately not about the actual facts themselves, but what the revelation of the information leads to. The discovery of dramatic information from an NPC source or the like (i.e., any situation where you're not simply told by a PC) is fundamentally a conflict. What's at stake is the consequences. This kind of situation should be negotiated by explaining to the other person what you're trying to achieve, working out the conflict at hand, and figuring out the consequences of the revelation. The other player can then tell you what you heard that led to those consequences. Note that the information that the other player reveals doesn't have to actually be true. The point is actually the drama, not the information itself. (Among other things, this allows players to genuinely keep personal secrets to themselves until they feel the time is right for a dramatic reveal.)
Note that players are still obliged to respect IC/OOC boundaries, and keep in mind that because information is a valuable commodity — it is, in many ways, one of the key currencies for scenes — one needs to always respect the rights of other players to control the release of the information that they have.
The Responsibility of Secrets
Secrets that only you know don't have impact on any play but your own. While this might be fun for a while, the real power of secrets is in their revelation. For the purposes of drama, secrets exist so you can share them with others. Usually, secrets are revealed slowly, to an expanding group of people, in a web of trust; as the secret spreads, it may eventually become common knowledge. We view this as healthy; as time goes on, new secrets are always created. There's no finite supply of secrets, any more than there's a finite supply of stories.
The power of a reveal (and by extension, a secret) is in the lookbacks. At the moment that a secret/twist is revealed, it has dramatic power — it makes the player who's learning the secret look back over the entirety of the narrative to date and see certain elements in a new light, in a rush of insight. The number and quality of these lookback moments correlates directly to the power of the reveal (and is sort of the inverse of foreshadowing). As the keeper of a secret, it's to your advantage to reveal that secret at a moment that creates this kind of dramatic power — and it's your opportunity to create the chain of events that occurs before the revelation that makes someone who later learns the secret to have a moment of "oh!" discovery.
If you're acting as the GM for a plot, you'll probably have an element of mystery in your storyline — investigation tends to be part of almost all plots. When you GM a plot, you want to keep in mind that, in the end, you want your players to understand what's going on — to solve the mystery and be able to take useful action. You want to create and maintain dramatic tension, by revealing enough information that the players stay on track that allows them to take useful action. You also want to make sure that your plot has a fairly high tolerance for unexpected twists and wacky solutions, because your fellow players are going to be tricksy and sometimes outright weird — "players do the unexpected" is an absolute truism, and if you frustrate the flow of information, there's a strong chance that players will either resort to the strange in order to try to gain information, or they'll give up and go do something that seems like a more entertaining use of time.
When you're GMing, you'll want to keep in mind whether the information you're giving out are "tells" or "clues".
A "tell" is a piece of color information that seems like it could be dramatic information — but it isn't. A tell is a hint that there's information to be discovered here; it should create curiousity, and be interesting in its own right as a detail. Usually, you'll drop multiple tells so a player will follow up on at least one of them. In order to move your plot along, you'll need to drop enough tells that the player knows where he ought to be looking for more information. The tells that a player pursues also tells you where his plot interest lies.
A "clue", on the other hand, is dramatic information. Discovering a clue essentially unlocks a piece of the story. Upon learning this bit of information, the character knows that he needs to take action. It's possible that discovering one clue will simply lead to an action that allows the player to discover another clue, but such investigative activity should be broken up with opportunities for debate and direct action. Every clue that you as a GM reveal should spur a player to go have a scene with someone else, and create a sense of urgency that something needs to be done (usually because inaction lead to something bad).
Keep in mind that, as a GM, the whole reason for having secrets in the first place is so that players can discover them. If there is a piece of information to be found, there is no question of if a player will find it; the only question should be when and how a player will find it. Your plotline and tells should be constructed so that the mysteries can be shown to the players, no matter how inept they seem at figuring them out. There may be consequences for character ineptitude, but ultimately, in taking on the mantle of a GM, you have volunteered for the noble and difficult task of trying to entertain other players. What you are trying to do is to create opportunities for players to act, and the dramatic tension that pushes them to act now.