I don't often look at Zoo Facebook pages, unless my attention is drawn to one by something particular written on here. Obviously they are designed more for general and prospective visitors, than those with greater specialist interests. They are really a publicity machine but can provide a source of occassional animal 'news' and information from time to time as well. For that alone I think they have some value to Zoochatter-types, but as you said, you need to take them for what they are, which is not as specialist information sites.
I do agree with you on the Facebook comments but must admit by some of the comments on here lately (all threads that is) I really can't see the difference sorry.