It would definitely be a while before that happened. I could see steem as a big contender to bigger platforms like reddit and facebook, but I'm sure that when it gets big enough it will either sell to one of the top competitors or become the top source for content in the world muahahahaha.
Whenever someone launches a website and thinks "I'll never sell it, even if it's making $1,000,000 a day!" there is still a thought in the back of their head saying "Well, if the price was right I'd sell :) "
It's too soon to tell if facebook will be taken over by steemit, but I'll be here for the ride and I will watch as this community grows into a powerhouse :D
Technically steem being blochain based belongs not to developers, but to us (to whales mostly of course) so to "purchase it" some big company just need to purchase lot's of steem driving price way up.
Though for it steem will need to grow at least to 10 000 000 users.