They have 4.5 messages and 6.5 to-do items per basecamp user account. That suggests to me that the number of active users is 10 or 20 times lower (100000 to 200000 users). Even if only 20% of those are paying, that's enough to be very profitable.

Given these numbers, I'd estimate that they'd serve 3-15 billion pages per year. With 100 CPUs, that's 1-5 requests per second per CPU. Of course, what matters is peak usage, not average.

Me too. The numbers look good. I presume they must be nicely profitable for those numbers. Unless the hardware and energy costs are too high (rails is a bit power hungry).

I wonder why they took on investment from Jeff Bezos - maybe it really it just to have involvement of Jeff Bezos (which would be entirely sensible) not to do with money.

Good on them I say.

From my personal (not very broad) experience and from talking to people who ran/developed free websites with accounts, a typical ratio of total accounts vs active accounts is 10:1, perhaps 10:3 if you are really good. On top of that (depends on the service) you may have a significant percentage of active accounts that represent the same user, but 10% of active accounts seems like a safe bet.

TB uploaded/downloaded numbers are really useless, they just thrown them in to look more impressive I suppose.

This means they probably have 20-40K of real active BaseCamp users.