I am in a similar position. Was about a week away from pulling the trigger on parse after a lot of research into the backend service providers. Now I will definitely look elsewhere. One problem is that FB APIs have a tendency to be constantly changing, and the documentation is not good. This shifting sands is not a good place to build a service.
However, the biggest reason is that FB is NOT a back end service provider. Facebook's customer is the advertisers. Before getting bought, Parse's customer were the companies that used their service. Now Parse is part of FB so future directions will all be in service of FB's main customer, the advertisers. I have no problem with this from a business standpoint, I am sure it is a good move for FB, but as a customer of back end cloud services I want to be purchasing that service from a company that puts me first. FB will not do that.
TL;DR: I was Parse's customer, I am not Facebook's customer.