Whoa whoa whoa, I don't think reddit has even close to 8 million active users. I'd say much closer to about 100,000.
The announcement section, which is included by default, and low traffic enough that I doubt anybody unsubscribes, has 300,000 users. Even if they're retaining 30% of their users (and keeping them as active), that still isn't even close to 8 million.
To put things in to context, reddit ran a haiti donation campaign which was heavily promoted through their ad spaces and blog postings and individual "donation drive threads" and they raised $185k from 3783 users over a period of ~30 days. They broke their first 100k within 12 hours. So if they got 6000 people to donate within 48 hours or so, that pretty damn good, over next few weeks as they add subscription only features its bound to get much much higher number as it gets promoted more.
Hmm, good point. The 8 million number comes from the blog post. Maybe the vast majority of people who visit the site don't bother to create an account?
The announcement section, which is included by default, and low traffic enough that I doubt anybody unsubscribes, has 300,000 users. Even if they're retaining 30% of their users (and keeping them as active), that still isn't even close to 8 million.