What do you mean by "Facebook could buy Steem if they would feel threatened" ?
Steem is a decentralised platform based on blockchain technology. How do you buy it ? Who is the person to look for to buy Steem ? There is no buying Steem.
I'm quite sure Facebook is already well aware of Steem. They have people doing only that (looking for treatening new disruptive social platforms). They are probably freaking out as they learn how it's built and they are incapable of stopping or buying it.
"buy" the idea. That's what I mean.
Which will be a victory for steemit!