YouTube has a social network behind it. That's what Google bought, because to date they have yet to build one of their own.
What a startup can create that a large company cannot steal would loosely be defined as a social network; mind-share among people who believe that your company is the go-to place for your service.
If you have built your service on top of Facebook, it will be very difficult to build your site so that you, and not Facebook, are what your customers thinks of when they think of your product. If it's truly a killer Facebook app, they will likely even be happier with a slightly inferior but fully integrated solution. Because they control the social network, the mind-share, that fuels your app.