It's like dating a very pretty woman. You can worry all the time that she's got lots of suitors and you can destroy your relationship by being jealous of all kinds of things that you imagine might happen or are happening. But the only thing that matters is that you're together and she apparently chose you, even if you're a bit insecure about why it is you over all those other options out there.
That would say more about you than it would about them ;)
Be happy, make them proud and I'm sure they'll make you proud as well.
To continue the analogy, big companies are women(or men if you prefer) in their early 30s worried they are gonna be alone forever. And you, your partners and they are all stuck in a bar that you can never leave.
Don't fret. View it as validation you've got a good team. For all of you, if your startup is successful the opportunities from Apple, Google and Facebook will only get better. If the startup doesn't go so well, those offers will still be there.
You just have to keep making sure the idea pushes forward as fast as possible. If it stalls or the idea becomes uninteresting then the easier it will be for them to jump ship.
Ask yourself if you trust your partners. If you trust them, then don't even worry about head-hunters and recruiters. If you do not, then you should not be in the business together anyway.
The fact that they've pledged their allegiance to the start-up in question speaks volumes about them and I think your suggestion completely misses the point here.