Their service is probably set up so the per-user overhead is low.
Think about it- what does your ‘using it’ actually consume? a few hundred KB of disk space and a little bandwidth?

I agree it’s a great value though. Signed up a few weeks ago and haven’t looked back.

I’d imagine their business and enterprise service is what currently or will pay the bills for them. Either way, I love their approach and the fact that it’s open source.