Twitter is a botfest too - post anything with the phrase SEO in it and you get a bunch of followers with no icon, no tweets and no value. This is where Facebook has a big edge, the focus on real identity and the crack down on API use has kept Facebook much cleaner than the others
I don't think it's the focus on real identity for facebook, so much as the segregation of the network. Since most status updates aren't public by default, you can't use the equivalent of search.twitter.com to find all recent people posting about a keyword. This has good effects (no spam every time you mention 'iPad'), but also bad effects (no easy way to find people you don't know talking about topics you care about).
It is not just SEO. If you post anything with the word iPad or iphone you get a bunch of spam replies (like six or seven) all promising you will win a free ipad. It has gotten so bad that I had to resort to writing it in l33t speak.
All you have to do is read the description of trending to see how people will try to game Twitter:
"Twitter's Trending Topics algorithm identifies topics that are immediately popular, rather than topics that have been popular for a while or on a daily basis, to help people discover the "most breaking" news stories from across the world."
If you want to up your followers, toss a profanity in every tweet. It's that simple.
Disagree it still exists on Facebook very heavily. You probably just don't see it in your circle of friends. So yes Facebook has an edge in the sense they keep the hordes of dead accounts, spam bots, etc. away from you.