Your Salesforce analogy also reminds me of Heroku - they sell you on the core product, which is hosting, but they have a large enough market that they can invite 3rd parties to develop apps (Add-ons in Heroku's parlance) for their users.

I think App.net is missing that "Killer App" that brings people to the platform in the first place. Perhaps they were thinking that would be Alpha, but it doesn't seem to be working so well.