From https://onesignal.com/about:

It's free; how does OneSignal make money?

We make money by using the data we aggregate to improve web and mobile experiences. We also offer custom solutions to enterprise clients.

Wet blanket! The rust will sustain them!

What does the expression 'Wet blanket!' mean?

It generally refers to something that ruins a happy/good situation -- like throwing a cold wet blanket on someone out on a summer day.

Thanks.