I think it often happens organically. You follow someone, you see...
· Comment — Sparr Risher's post · View on Facebook · Markdown source
I think it often happens organically. You follow someone, you see someone make insightful comments on their feed,so you follow them. They see you've followed them, so they take a look at your profile, and if they like your recent posts, they follow you back. They're also often friend's groups in real life.