It's a pity they have us so whipped into their mindset. Because, while they define themselves as a social networking site, their definition of "social" appears to be based on Victorian standards, because by their existence, social networks have completely changed the definition of social. When you strip all the definitions down, they add up to "people who socialise together." And even several millenia ago, thanks to messengers and messages passed back and forth, some people already socialised together without ever having met.
With the coming of mail post, societies could become more geographically diverse, with the telephone, even more so. BBS systems created social groups spread out all over the world, and with the Internet, our "social circles" include circles with astronauts in orbit, and even a small robot on Mars.
In this sort of environment, the injunction to stick to people you know seems a bit small-minded, petty-fogging, limiting, and counter-productive. Why do I use SNs? Because it enables me to meet people I would otherwise never get the chance to meet. I get to give and receive ideas from some of the smartest and most knowledgeable people in the world, no matter where they are. Limiting me to only the people I already know is a bit like saying "only use your telephone to call other people in the house, no outside calls..."
Isn't it time the "social" networks redefined themselves again?