When marketing your app, bullet points count, especially in the App Store description and for reviewers. To that end, what you can and cannot market as valuable social features should be noted.
As you've seen, social promotion of your app is important, but throwing in Facebook Connect or (as in iOS 5) the capability to Tweet events or share achievements does not make your app a “social” app, or encourage users to download it on these merits alone. Neither does the capability to promote it via e-mail, though it is a feature most apps should have. These types of social interactivity are extra features that can both help promote and extend its value, but they hold little value as bullet points for marketing these days, because they are both simple to do, and most apps feature some sort of social connectivity (or are trending that way).
Social features, on the other hand, are meant to be marketed and called out in your App Store description, whereas social extras might be the last bullet point, but certainly not the first. A social feature would be the capability to directly go head-to-head against or play along with friends, in-game chat or message, share photos, or collaborate. As you're developing your app, see if you can implement social features that provide more marketing mileage, versus social extras, which should be included, but are generally less meaningful.
Because apps with a form of competition, cooperation, or collaboration ...