"What is it about this completely voluntary relationship that is so inherently evil?"

I'm not going to argue it's inherently evil but,

You are right that the service is completely voluntary. However, the opportunity cost associated with not having a Facebook account has been rising by virtue of the network effect and 3rd party services requiring a Facebook account to access functionality.

The fear is: as (if) Facebook becomes more and more a part of society, the cost of not having a Facebook account becomes high enough to make it practically compulsory to have a Facebook account. And if at this point Facebook acts as it does now, well then it's time to start worrying.

Picking up a hoe and tilling a field is completely voluntary, so why did slaves choose to do so? Because not doing so was too costly.