Don't act like you don't care.
Okay, nobody cares, but I'm going to tell you anyways.
Twitter! I have the 'you have a new follower' notifications turned off, so I don't know or notice when new people follow me.
When new people TALK to me, I notice, and I try to always ALWAYS follow them if they are talking to me and follow me.
I follow new people who DON'T follow me if I know them in real life, if they are news source people, or occasionally otherwise, but very rarely.
I unfollow anyone who pops up into my stream and I think, "WHO THE FUCK IS THIS?" or if they post something really insufferable and I don't know them all that well to begin with or if they only post links to their blogs and never actual content or if they think twitter parties are a normal thing to participate in or if they unfollow me or if they say something that I find super offensive or just makes me think that they're an idiot or if they auto-DM me or if they ask for votes in any sort of internet contest more than once exactly or if they send me skeevy messages and act like they want to see my lady parts or if they bandy about the C U Next Tuesday word in a way that offends me OR (more often than you might think) twitter unfollows people FOR me and I won't realize because it's madness in there and one day I'll see a retweet or other mention of a person and think to myself, "Why didn't I see anything that she (it is usually a she) posted OH LOOK I'M NOT FOLLOWING HER I DIDN'T UNFOLLOW HER," but if that happens I refollow the person and just hope for the best.
In sum, I will follow you if you follow me, interact with me, and aren't a twat.