Devan phrased it well (using his elite new irc nick):
12:39 <@jbowes> facebook is way too much like orkut
12:40 <@jbowes> or all them other ones
12:40 <@jbowes> why is it that people seem to like it more?
12:40 < kryptos> it tries to replace everything
12:40 < kryptos> we have email
12:40 < kryptos> we have blogs
12:40 < kryptos> we have flickr
12:41 < kryptos> i wouldnt even be on facebook if i didnt have to check email for plans tonight, and constantly see facebook emails
12:41 < kryptos> maybe i need to filter them to trash
Facebook wants become the canonical source for your data. It keeps you coming back with its stream of email. This is reminiscent of the instant gratification aspects of gambling and other addictive activities.
If you think I'm being paranoid and over the top, check out this Boston Underground article.