Products need competition. Parse was the only system I'd have considered "similar" to Firebase and with Parse as a service gone, regarding realtime databases, Firebase was all that's left. With this available, at least theres an option now.
Regarding SocketIO I don't think it impacts it at all. If you like using SocketIO, continue to use it. Firebase negates the need for that. If you like using Firebase - continue to use that.
I'm a big fan of Firebase but 1 major pain point of mine is the fact that it's a BaaS. I don't mind 3rd party services for non mission critical parts. If Mailgun or Cloudinary or Algolia go down for a bit of time, yeah - that sucks, but it's not gonna make my app come to a crashing halt. If Firebase goes down, if they decide to close the doors 1 day (like Parse did) - that will hurt me. They (Firebase) supposedly have some major announcements at Google IO this week. Will 1 of them be a self hosted system? I hope, but I doubt it.
If I can find some time to play with HorizonJS and it looks like it can ween me off of Firebase, I may just take the plunge. As of right now though, it doesn't look like their docs support AJS 1.x and I really don't have the time to rebuild in React or write my own connector.