Fanout vs PubNub vs Socket.IO

We are starting to work on a web-based platform aiming to connect artists (clients) and professional freelancers (service providers). In-app, timeline-based, real-time communication between users (& storing it), file transfers, and push notifications are essential core features. We are considering using Node.js, ExpressJS, React, MongoDB stack with Socket.IO & Apollo, or maybe using Real-Time Database and functionalities of Firebase.

I am building an IoT application that will utilize connected air quality sensors to provide real-time indoor air quality in offices. I want to be able to share this data with a few different databases, etc.

Wondering if anyone has any advice on which real-time streaming API would be best for this sort of application, or even how I should think about it?

It really depends on how you want to do it. If you use Socket.IO, you do have to have your own server running somewhere else, a good free one is Heroku. Pusher is nice if you know what you are trying to do, know your limits, and how to do it. While Socket.IO allows more flexibility if you need to do something crazy, Pusher is great if you're just doing standard real-time data transfer because there is minimal setup. The only other thing about Pusher is that it is not entirely free, so if you know the pricing, know how much you are going to use, and are willing, you could easily use Pusher.