None? Why is 'social' stuff so cool this days? I still strongly believe word of mouth is the best way to share what you believe is worth showing. When you browse the internet you are constantly bombarded with things a friend of a friend of a shared that you do not really care, but if someone you trust told you personally to check out a website, the you check the fuck out of that website. Therefore, is a social integration really needed? If so, why?
Shallow social integration like "tweet this" or "like that" is crap, I am really not interested in that. What I am really hoping to build is an app that users can connect over their friends with.
I guess there are two reasons for social integration: promotion, which involves broadcasting the users participation to his circles (and I care less for that), but the other is engagement (come back to the app because the users friends did something interesting there). Games like Draw Something or Words With Friends for example are fun even when you aren;t tweeting actions to the non-participating audience. But its a lot less fun if its just a game you play with the computer itself.