Ask HN: Easily adding social networking features to an iPhone app?

I was just looking at an app which integrates social features really well, and thinking "man, I wish I could add these features to my app without a lot of work".

Stuff like:

- read and add comments for each piece of content

- like/dislike/add favorites

- friends list

- profile with a wall

Basically very simple Facebook-like features - as light-weight as possible, because if they're more heavy-weight they're harder to integrate well.

Does something like this already exist? I guess a combination of a library + hosted service would make the most sense.

If not, is this something developers would consider using in their apps? How much would you be willing to pay? :)