Facebook addiction is a pretty big problem when it interferes with your real relationships. People in those situations feel that they aren't worth connecting to if you can replace them with a social network, or that you're looking for other people to connect with instead of the person next to you.
Anyway -- I like Pinterest and Instagram. I'm, admittedly, not the typical Pinterest user - but it's kind of a large snapshot of all resources on the internet. Recipes, home decorating ideas, art, and humor. I find many websites through Pinterest that I end up being a loyal subscriber to. Instagram I really enjoy to see what's happening around the world at any given time. While I'm posting a photo of what I'm doing, I can simultaneously see what's happening in Africa, Russia, Japan, or on the train passing by my house. It's an awesome glimpse into a realtime view of the world.