I have a few friends who post those "...if you agree, repost this" statuses all the time. Many of them go even further with "90% of people won't repost this, will you?" and other bullshit meant to challenge me. How do you know, before it's even happened, that 90% of people won't repost it? Could it be because 90% of people have, in the past, demonstrated that they're not sheep?
I think I reposted one of them, once. It was something I agree with, so sure, whatever. Then I noticed them coming more and more often, and I had to make a "rule" for myself never to repost a status. Never. If I agree strongly enough, I'll "Like" it. That's all they're getting from me.