They could influence elections, shift social policy to help certain people, and generally engineer the social world in their own image. It will be very hard to tell if this is happening.

They could become the whole internet. In many places they already are. This is a very sad future for someone who sees the internet as a tool of human liberation and direct connection.

I don't care so much about them selling our information. It's worthless even to me. These other things are much more subtle and valuable.

They can make users miserable by messing too much with them or trying to squeeze more money out of them