The stat listed on our site is for a typical website. If you have a mobile app, or an atypical website, your mileage may vary. Even for highly engaging, paid apps I suspect you'll find it's in the 100 to 1 range (or more). 1 CCU = 20 MAU is very high.

For example, World of warcraft peaked at ~10% of their users online at any one point recently: http://venturebeat.com/2012/11/07/wow-pandaria-1-million-con...

Also note that we bill at the 95th percentile, not peak usage, so you can spike well above those numbers without consequence.

If you want to know how many concurrent users you'll need, sign up, drop in our script, and have your clients connect to Firebase (but do nothing). You'll be able to see concurrent user counts under Analytics in Forge.