There are many users for which it is not fun, but an addiction. I have spoken to lots of people who say about once a month something like "Yeah, facebook is really bad for me, I just waste time and get upset", but they can't stop checking it every hour, and responding to posts that touch them emotionally, in either a good or bad way.
That's by design, of course - it benefits Facebook greatly that its herd is addicted, and unlike people addicted to alcohol, nicotine or other substances - there's not even another supplier they can turn to: It's either feed your addiction or suffer withdrawal symptoms.
And I think the success rate of quitters (as a percentage of those who actually want to quit) is also comparable, at single digit percent.
