I think it's something of a joke to think that Steem could represent a real challenge to Facebook.
The minds platform resembles the experience of Facebook much closer, has a similar promise of rewards for contributing to the forum, and even both combined user bases doesn't even make a dent in Facebook.
i doubt steemit could make a dent the way it is ... but it's actually Eos / block.one(?) Voice claiming to go after facebook and mister Justin Sun announcing a big announcement on the day the announced themselves and then this happens ... so best guess is he wants something against what he maybe sees as one of his big rivals and this thing here is already well-developed (software-wise) and probably came at a bargain , lol, but i doubt we'll ever see the details of that deal in public in the land of decentralization-mythos , in the end the only decentralized thing is still the classic chains with their thousands of nodes but even there its a select group developing.
IF and thats if steemit were an actual 'thing' and it would be anything to facebook, not even a threat, then zuck would have bid on it, just like he did instagram
the fact that he never even blinks at it says plenty ...
Atari (back in 'the day) killed themselves trying to be Sony or Nintendo while they simply could have been Atari
competitive psychopaths (i think they call them CEO's or something) can be freakishly weird people with strange notions about winning to the point of self-destruction (with ample collateral damage :)
Agreed, I know it's not accurate to conflate steem with steemit, dtube and all but you seem to have caught the intent.
Though, I believe that Zuck had no interest in an established crypto token that was decentralized and open source. He had made a statement (or more) about making a facebook coin (not sure what happened since), that goal implies that, rather than buying something that would be open, that he was going to create his own coin tied to facebook in that sense would crush the intent of steemit (and minds).