If you reverted ownership there's no way that FB are going to sign that contract (a small risk you could inadvertently lose the asset and the cost price, eg through an unforeseen loophole that favours the seller - lawyers should veto such things, surely).
Also, are you going to make it a perpetual term applied to all future owners? If not then FB can probably make an entity to sell it to. Or use a third-party login that itself requires Facebook login and workaround your selling constraints.
I like the idea of it: just practically I can't see how it would be workable to technically constrain a company in a contract of sale of that company.
Are there examples of where this has been done successfully?