I’m going to join what seems like the rest of the blogosphere in congratulating Zuckerberg and Facebook on reaching 500m active users (see Zuck’s post announcing the milestone here). That is a phenomenal number of people and it is an amazing achievement to get there in six years.
It is also worth noting their incredible track record of innovation – e.g. news feeds, the app platform, Facebook Connect, the Like Button, and their ad-targeting platform, and the quality of the tech team which has kept the site running at blistering pace throughout their growth and innovation (contrast that with Twitter).
And they are a proper business, with revenues estimated at $1bn+ this year, and still growing at near 100%. From the bits and pieces I read I think they are near to profitability and I would expect that if they chose they could dial back growth and become profitable immediately.