1/ parse wasn't a core service for facebook, nor a relevant source of a revenu AND their API wasn't standard. Those points combined made it very risky for people to use it.
2/ since they open sourced their API now, and the service was a paid service, there's a very high probability that someone will very soon create a 100% compatible PAAS.
3/ firebase will be next to shutdown. Not because they suck, but simply because they're exactly in the same case (proprietary,non standard, critical technology, held by a company who don't really care about it for its money service). People won't sign up on firebase anymore, so they will have to shut down quite soon.
4/ Every non core project run by facebook should be looked upon with extra caution now. Yes, that includes occulus. They're clearly not going to sell many devices, and gaming console maker (aka Sony) will have the lion's share of the gaming market. So, expect fb to shut it down in 2 or 3 years if it doesn't get a very large marketshare (the only use case large enough i can think of that is not gaming being porn).
5/ I personnaly would be really hesitant now to run on something like google app engine. I wouldn't be surprised to see google and microsoft moving forward api standardization for core cloud services on a much faster pace. Except amazon, nobody is really safe now.