I built my readership largely through myspace. (Of course, this was before myspace was dead, so I guess now you'd want to use facebook if you tried it.) However, my target readers are teenage girls, and I think myspace was a perfect place to find them.
What I did was to buy a myspace bot (which you're really not supposed to use) and then go and find authors who wrote similar stuff to what I did. I stole their friend lists and sent out friend requests to everyone them with a little note saying, "Since we're both friends with XXX, I thought you might like to read my book."
This resulted in a bloated bunch of myspace friends (about 5,000) and maybe 200 readers. But the readers I gathered were loyal and willing to tell their friends.
Some people would advise against doing this, because they say it's spam and will make people hate you. I didn't get very much flack, however, and I think it's because in my messages, I established a connection with the person right away. I told them where I found their profile (the author's myspace page) and that I thought they might be interested in my product because of our shared interests.
Still, I notice that the only friend requests I seem to get on myspace these days are from authors or bands. It seems less of a social networking site and more of an advertising site in my opinion.
Honestly, I think the key is to get a small base of readers and then just provide a quality, consistent product. Personally responding to messages doesn't hurt either. I think word spreads when your stuff is good.