Whats so special about Facebook and YouTube?

I'm back to my favorite subject Facebook and YouTube. What I was thinking about today is why did these two sites become the giants of social networking and streaming videos? How did this happen? I remember when I was in middle school and high school, there was a very popular website within the Russian community called www.whimit.com, it was in essence a "Russian Facebook", of course you did not have to be Russian to register, but the site was created by a Russian couple from Brooklyn and was marketed towards Russians. The thing that I'm wondering is why didn't whimit, or any other social networking site become as big as Facebook has? They have been around longer than Facebook, it's not a very original or unique idea. In fact as we have all heard the creator of Facebook, was accused of stealing the idea from his friend and former partner. The only thing I can think of that could have set these sites apart from the rest is luck. Anyone have any other ideas?