Facebook reported that they grew with 50 million accounts in the last three months and now have 250 million accounts. That is an amazing growth rate, and probably means that Facebook passed Wikipedia and now is the fourth largest global web site. Reporting the news, TechCrunch asked when Facebook will hit the 1 billion 'milestone' and if it will beat Twitter to it. I'm not sure that is the right question.
In May roughly 1.1 billion people had access to the Internet globally, according to Comscore. Reaching 1 billion users is having a little of 90 % of all Internet users coming to your service. According to InternetWorldStats global Internet users are around 1.6 billion, which would make reaching one billion easier but in no way easy.
How big would Facebook be relatively to other large sites if it had a billion accounts? Comscore doesn't measure accounts but rather use a panel to estimate the number of unique visitors (and other metrics) per month to sites. In May the global top 5 web sites measured in unique visitors were:
1. Google sites incl Google.com and YouTube: 840 million
2. Microsoft sites incl MSN and Bing: 682 million
3. Yahoo: 570 million
4. Wikipedia: 317 million
5. Facebook: 316 million
Twitter had 37 million unique monthly visitors in May making it a medium-sized web site. (Even if you use Tweetdeck or a similar application to update Twitter, I suspect that you would visit Twitter.com at least once a month and thus the monthly unique visitor number is not unfair to look at.)
Twitter might, for good reasons, be TechCrunch current focus of attention, but the relevant question isn't when Facebook or Twitter will pass one billion monthly users and have become the largest web site in the world. It is when Google will do it.