TLDR; Firebase, Parse, GoInstant, and other BaaS have similarities. There is overlap in the problems they can solve, but specific strengths to each. GoInstant is specifically focused on Multiplayer Apps.
Firebase and Parse are both awesome products. They are really useful as application backends. GoInstant is focused completely on multi-user, real-time data. It compliments your backend, rather than replacing it. All of the decisions and concepts in our API are there to make it really easy to manage collaboration between users. It’s completely focused on the idea that you’re developing for a multi-user use case, either sync or async collaboration.
So, everything that happens with GoInstant involves other users. If you set data in our key/value store or send a message, other users are listening to that. Everything is wrapped in a pub/sub msg queue. When you connect or disconnect, other users know. Of course, you want to decide which users will know, so we organize groups of users into "rooms". We've built it from the ground-up with enterprise grade security, so auth, access control, and encryption are all rock solid.
As a quick example: if you were building a shopping cart, you would store the products in a backend somewhere (maybe Firebase, Parse, or a relational database). If you wanted to make that shopping cart multiplayer, i.e: multiple users shopping, adding products to the cart, and checking out together, you would use GoInstant for that. It's not that you couldn't accomplish it with other tech, it's that we're specifically built for multiplayer apps, so it's going to be intuitive. Who's looking at which product, what page they’re on, who added what to a cart, who's editing the quantity field: all of these features come out-of-the-box with GoInstant.