Multi-billion what? Revenue? Yes, because they managed to monetize their traffic and keep it up despite fierce competition (attempts... by Google, for example). Market-cap? Yes because enough people who aren't typically extremely careless with their money invested in FB.
> All of their success comes from the luck of being first
First at what?
They didn't have to do anything, that's the point. They have such strong network effects. They would have to completely fuck up to lose even a tiny fraction of their userbase.
Apps. Messenger. Instagram.
They didn't get that network by just being first(ish). They got it by continuously being the best of the imperfect options, and they can lose it just as easily.
Look at Blackberry and Nokia. You don't necessarily have to fuck up, it can be you're simply not moving fast enough.
Flickr, Myspace, bebo, there are plenty of companies who were early and lost their advantage or went out of fashion.