Answer: Less than 21 million.

That’s the findings of a new report from Business Insider that looked at the total number of Twitter users that were following 30 or more people, which is the level that Twitter themselves uses to rank an account as active.

And if true, that’s a pretty shocking state of affairs for Twitter, especially when held up against Facebook (or even MySpace), and could have huge ramifications for the viability of the network as an advertising platform.

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Some massive stats were revealed by Twitter co-founder Biz Stone at the official Chirp conference today, including the magic number we’ve all been waiting for – Twitter has 105,779,710 registered users, and is adding around two million new profiles per week (300,000 per day).

No idea how many of these are legitimate accounts. You know, the kind that actually updates semi-regularly, and isn’t one of a dozen or more operated by the same individual. And bots do not count.

Active (and real) users is really what it’s all about, although even that doesn’t factor out the duplicates.

But assuming this figure is accurate, it’s worth observing that this means Twitter is already about one-quarter of the size of Facebook.

Other figures of note:

• The site is seeing around 600 million searches per day, and expects to see a ‘one billion queries day‘ by next month
• Twitter receives 180 million unique visitors per month
• 75% of all traffic comes from outside Twitter.com

The latter statistic isn’t too much of a surprise, but it doesn’t stand up very well against the one before it, at least in terms of the data we’ve seen in the past.

Compete, which mostly tracks US data, indicates that Twitter.com seems to have settled around the 21 million unique monthly visitors mark. Assuming that represents just 25% of visitors, this would indicate an overall global visitor/user number of approximately 84 million per month, which is just a little shy of Biz’s numbers, unless he’s factoring in API calls.

Maybe it’s the nineties again and he’s counting hits? Some proper data would nice. Still, at least it gives us something to work with for now.