I think it's the lack of algorithm that does that, more than anything else. I've noticed that Facebook seems to show my posts primarily to those most likely to be angered by them, rather than to those most likely to appreciate them. And then because the algorithms are also built "the rich get richer" style (those with more interaction stay higher in the feed) that feeds into itself.

I miss the time-based feeds (esp. on Pinterest) that allowed little-known content to be discovered if you logged on at the right time to see it.