They can influence and censor people they don't like. You see a lot of this going on over at /r/ethereum, where certain submissions receive high number of upvotes but little to no comment activity apart from obvious sock puppetry creating fake conversations. Comments pointing this out are quickly downvoted to oblivion or flagged.
The anxiety from Reddit is obviously clear. Their user base premium is losing it's value from an investors point of view. Mindless banters you see on Youtube are what makes up Reddit's audience with occassional anecdotal experiences that show up.
Pornhub has ton of users but are they valuable as Facebook users?
I feel like there's going to be a market correction in the quantity over quality type of social network websites who've ramped up userbase but have not extracted tangible value apart from delivering shaky metrics that advertisers are now beginning to raise questions.
It seems like the only ad platform that stood test of time is Adwords but as a small advertiser I'm turned off by the prospect of paying $1+ per click.