I challenge you to follow up on it. Block them.
Let me give you some anecdotal background story...
I realized this was happening to me a few months ago - that I was wasting more and more time just surfing Facebook or Twitter and so forth. So I started adding websites to my /etc/hosts file. (That is, my computer now blocks me from accessing these websites, unless I edit a file, and then restart).
It started with Reddit - one day I read an article on Reddit that said something along the lines of "How many of you are intelligent, but waste a ton of time on the Internet?". It struck me that I was one of those people - that my potential was dwindling away because I was wasting tons of time on Reddit rather than getting interesting things done.
I blocked Reddit a bit later, figuring I might unblock it given time, but I never have. After the first day or two, the instinct to open up a Reddit tab just died. I still feel an attachment to the community, and I feel kind of bad that I left, but I don't feel that I was addicted like people get addicted to drugs or so forth.
Later on, I blocked some other sites that I frequent, including Facebook. I even blocked HN for a while, but I realized that I wasn't wasting nearly as much time on here as the other sites, for some reason. Perhaps it's because the rate of change of articles is small. Plus, HN has noprocrast settings, which are great. Anyway, I digress. Sure, there are disadvantages, but the advantages, and the savings in time, are huge. I know a lot of people say they don't want to leave FB because they'll lose contact with people. 2 responses: One, my last FB status says to contact me on gmail - so if anyone really wants to get in touch with me, they know how. Two, it's only blocked on my computer, so if I really want to take the effort, I can hike over to a library computer or something and respond to people.
Now, the point to this comment is to convince you that you have to block these websites too.
There are two things that I would want to tell myself in order convince myself that I needed to go through with it.
1. The reason I didn't want to do it - and didn't do it, for a long time - is because I always thought that I should just have enough will power to stop doing it. If you believe this, remind yourself that even though you believe this to be true, you keep coming back to Facebook every few hours or days. Why is that? It's because even if you have enough willpower 90% or 98% of the time, that small fraction when you don't is enough to open a tab to FB, but once you open a tab, you could end up wasting an hour.
2. You become more aware of how you spend your time, and you become more productive, after you stop wasting it. I used to dump hours into just surfing the web. Now I'm quite sensitive to how much time I spend, which is advantageous. Plus, most of the sites that I visit (like HN) require constant focus, so you can't really get into that vaguely unfocused mindset that allows you to waste tons of time on things like Facebook.
In short - you said that it had to stop now. I challenge you to follow up on it :) Blocking FB, etc, was an awesome choice on my part. I think it will be for you as well.