I have no problem with the fact that my car engine is molded or the fact that my plugs are too.
But the same is true of closed-source. Windows is closed source, and yet StarDock is able to completely alter how it works. DirectX is closed source, and yet large numbers of graphics drivers hook deep into DirectX to alter its behavior. Even anti-virus companies hook and modify bits of Windows they don't have source code to.
You fail on both the fact that closed source things do get modified by people without the source code and the fact that many open source things don't get modified by people that do have the source code!
Using FaceBook to show how great open-source is is like using a 32lb steak to show how tasty being a vegetarian can be.
Facebook is perhaps the least open thing on the entire Internet. It's a walled garden, which doesn't open source its own code, which hoards user data and even has its own languages (FBML instead of HTML and FQL instead of SQL) that it has no intention of ever sharing.
You are stating the advantages of public/open APIs, which we seem to agree are better then closed APIs. But not as good as being fully open source.
Facebook built their company on open source. That's what I'm getting at here, not contributing to open source. Certainly not the idea of giving away all your source code, or some kind of computer science utopia where all technological advancements are open.
But really, to use as much open source as you can to leverage your own unique service. Facebook does contribute a lot back to open source even though they don't have much of a legal obligation to do so. It's often good business to do this. But giving away everything is rarely good business, sometimes it can work but most of the time it won't. The unique elements of Facebook that give them a competitive advantage will probably never be open source, and that's fine.