Unless your character had his home town burned down, or you started the game at the birth of your character. Your character will 'know' people from 10 to 500+ years of their life before the game started.

This is full of "black holes". You're missing out on years of experience of interacting the 'known' character that you're introducing. Which means quite simply put that if past experience is relevant(which it almost always is to some extent) you can't let the player know without blatantly telling them. For instance this leads to some really retarded conversation: the first option being that you tell X 'known' character that you're interacting with that "I don't trust you"(this is of course stupid since if you're telling someone to their face I don't trust you it's the same as revealing A LOT of your hand), likewise if you end up telling your "party" not to fully trust someone a real party would likely vote not to work with the guy(after all we're fighting/running from/whatevering powerful enough enemies to have a story worth telling, and we're fighting monsters only from fairy tales every other day). Other plot holes which are classic in something like final fantasy is that you the main character finds out that the reason you're going here because there were people you knew already(a surprise that shouldn't have been since it was the player's plan). Or worse case scenarios where the player can trigger plot/sub-plot points by doing something that they 'should' already know(but is actually a surprise to the player).



So what are some classic and not so classic ways of dealing with known-unknowns, though mainly from the aspect of meeting people.

I think starcraft dealt with this pretty well. The idea is that the hero characters aren't really the player, even though you sort of play as them, so they have thier own personal history and such.

Another way is have a bit of a prelude to introduce the important characters. Like you start playing when you are really young, and then cut unimportant years until the relevant stuff. Fallout 3 and Fable did it like this.

Another way is that the player is new to whatever the setting is.

Having the character lose thier memory before the start is a reliable cliche.

Now a days, no one makes manuals anymore :(.
There were a lot of games that dealt with the issue by giving you tonnes of reading to go along with the game. Starcraft had a lot of back story to the setting in the manual. Homeworld had tonnes of setup and thought out history. The Warhammer 40K games are based in a universe that has multiple stories, guides and books as backstory.

Games like Age Of Empires, Empire Earth, Call of Duty, and try to draw off real events in history to create their scenarios. You have everything from ancient history to retellings of modern events. The game relies on you knowing about your real life world.

Games like the Indiana Jones adventures or James Bond games draw off an established "hero" character. You know that your hero has lots of allies, and that not all of them are useful now. Directly telling the player to go see one isn't out of character once you reveal the purpose. The hero is deciding to act on information he already knew about, but wasn't relevant till the moment at hand. This can also be done without an established hero by having the main character narrate their internal monologue to the player.