But surely this is a two-way street? If Facebook becomes less valuable to me as a website operator then I have less incentive to keep any Facebook integration

But surely this is a two-way street? If Facebook becomes less valuable to me as a website operator then I have less incentive to keep any Facebook integration on my site, and if I remove that then they lose the insight into my visitors that this currently affords them. That wouldn't bode well for their idea of selling ads based around users' off-site activity, would it?