App.net's API does providing more than a microblogging API. They have a file hosting (each user has their own quota), permissive messaging (point to point or group), and a search API so you can easily find anything on their system. They're planning to add more useful APIs, such as billing.
Seems like bad business strategy to build on top of those if it means it's easy to steal away your userbase. Sounds fine for mid-size apps who can't justify rolling their own though.
If your only competitive advantage is user lock-in, then you already have a problem. Either you need to get your userbase to grow big extremely fast, or you've already lost.
And if you grow the user-base fast you've won, right? Lock-in and network effects matter enormously. Saying you can't lock-in the social graph neither guarantees a company won't lock-in elsewhere nor guarantees this is an appealing platform on which to develop.
This isn't a morality tale, I'm just pointing out what kind of decisions would be made around App.net and that the Vine/Instagram example doesn't really make sense.
Unless you're going to provide free infrastructure for every component (not just the social graph) there will be an element of lock-in to virtually any social product.