I have this Google+ account where I post almost nothing, and yet people keep adding me. That puzzled me until I realized that the cost of adding someone who doesn’t post anything is almost zero because they will not pollute your feed anyway. There is a cost only to add people who keep posting garbage.
As far as interesting people are concerned, the more interesting they are (whatever that means) the more followers they tend to have. Also – up to a certain point – the more interesting posts per day they have the more followers they tend to have (because they increase their chances of being reblogged by others and thus they increase their visibility on the network). However, if they post too many things per day they’re beginning to be annoying and there’s a point beyond which they post so many things that they are all over people’s feeds crowding out other people. Beyond this point people get upset and unfollow them.
The story in a single picture: