What are you using Realtime databases for?

Are you using realtime databases such as Firebase, AceBase, RethinkDB, etc? If so, please let me know in the comments:

1. Which realtime database(s) do you use?
2. For what kind of functionality do you use the realtime aspect?
3. Are you missing features in those db's, such as indexes / query options?

I would greatly appreciate your response, thanks in advance!