Are you going to migrate your apps to some other platform?
Firebase is pretty cool and really easy to get up and running with, but I'd still be hesitant to use it in any serious production application (they were acquired by Google not too long ago, and it's not crazy to think a similar situation may arise there). Apache CouchDB would be my bet.
Facebook and SalesForce have announced to work more closely together. I think Heroku + Parse open source server will be the logical replacement for Parse.
Best strategy will be not relying on another hosted BaaS solution. Just because of this we have created an API generation tool called API Plug that can benefit mobile and backend developers. You can export MongoDB from Parse right now. After that you can generate & download your own REST API source code in any language you need in minutes from apiplug.com Then you could deploy on your own server and have the full control of your backend.
Vasan Subramanian
Previous generation techie
600,000 apps on parse.com, that's a lot.
I am tempted to replicate parse.com and offer it as a fallback service myself and rake in all the moolah ...