The problem here is that you have no visibility into Facebook's roadmap, priorities, etc. Sorry, but I think betting $5m (or years of your life) on a startup that can literally be killed overnight on the whim of someone at another tech company is crazy.
It's pretty obvious that a business that's completely dependent upon FaceBook isn't in a very good position. It's equally obvious that a business that's completely dependent upon FaceBook but has users is in a better position than a business that nobody cares about. You start with the latter, not the former.
If Greplin or some other startup is still dependent upon FaceBook after years gone by and $5M spent, they're doing something wrong. But in the near-term, they're doing a helluva lot better than the average startup.
In general, we're young in the space of companies that rely on APIs and other companies (App Store much?). Zynga has made a pile of cash relying on facebook, and plenty more will too, and that's just the way the world is from now on.
As for the whim of some individual, you are absolutely right. But that will sort itself out in the next few years, and it doesn't seem like a good reason not to get in early on what is obviously a trend that's here to stay.
