Yeah - I ended up digging around from several sources and realised I was only scratching the surface.
Here's some of the numbers I found - Probably the most useful metric was sign-ups PER DAY, so that's what these are. Sources are a mixture of companies' own sites plus wikipedia and so on. So don't take these as gospel - just a rough guide.
Figure in brackets is amount of years stats came from. So you can read it like Skype has been getting an average of 186k sign ups per day, every day, for 5 years...
Skype - 186,000 (5 years)
Facebook - 126,000 (2 years)
Myspace - 93,000 (3 years)
Age of Conan - 55,000 (<1 year)
alladvantage (agloco predecessor) - 18,182 (2 years)
barbiegirls.com - 10,000 (1 year)
World of Warcraft - 9,000 (3 years)
Xbox Live - 9,000 (4 years)
Warhammer - 8,000 (<1 year)
Friends Reunited - 8,000 (5 years)
Joost - 2,000 (2 years)
Age of Conan 1,000(2 years)
The point of the research was to bring some realism to the sign-up projections some of our ideas.
Idly dreaming of 1000 sign-ups per day for a great idea is great - but this limited research suggests that you have to have something mega-viral (facebook, skype) or something that's already a big brand (barbie, Warhammer) to get those numbers.
NOTE - I'm quite interested in Agloco but found it very difficult to find out how many people have signed up to their scheme. Anyone have any idea?