Its free advert for crypto. Many people use social media and a single crypto ad will bring huge exposure at a less cost and most efficient way. Bounties utilize this. But of course, as fast as social media can help in increasing a project's popularity, that's also how fast a fake/unfavorable news destroys it.

There are many ICOs that aim to tokenize social media, one example is tipper. This brings more application for blockchain. I'm not sure though if it has any significant effect to already existing social medias like twitter and facebook.