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 200

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.