Could it be an actual replacement of facebook?
I mean, with private messaging included?
I see blockchain as an open ledger. Which makes it not suitable for 'private' messaging :)
Of course, there's the option of encrypting messages.
Then again.. What about performance. Blocks need to be validated, which takes time...

So, I would think a mix of both worlds will be needed to actually replace the current centralised platforms.
Probably we need to get over the hype curve to see those thinks clear? Not everything has to be put on a blockchain I guess ...