Facebook may be the subject of all of the headlines with its public debut looming this Friday, but another major player in the social networking space is reminding the world that it’s still growing too.

Twitter, which has built a company that one day might go public too on the back of 140 character messages, has waived its hands in the air by announcing that it has surpassed 140m users worldwide.

As reported by The Guardian, 10m of those users are in the UK. That’s good enough to make the UK Twitter’s fourth largest audience behind the US, Brazil and Japan, and explains why the company has a 30 person strong office in London.

Although Twitter’s userbase can’t compete with Facebook’s, the service’s impact on society has arguably been nearly as significant, and in some areas, perhaps even more significant. As The Guardian’s Charles Arthur notes, “over the past year [Twitter] has been blamed for inciting riots – a charge that was disproved – and of undermining superinjunctions involving, among others, Ryan Giggs and Jeremy Clarkson.” And, as Arthur points out, Twitter has become a key platform for prominent figures, celebrities and brands to interact with the public.

Like Facebook, Twitter has a front-row seat to the mobile revolution. According to the company, some 80% of British users who used Twitter in the past month did so using a mobile phone. Obviously, Twitter’s service, which was inspired in part by SMS, is more easily adapted to a mobile experience than, say, Facebook’s, but in terms of monetization, which Twitter has taken slowly, the dramatic rise of mobile usage is clearly going to present challenges for Twitter too.

One challenge Twitter is facing that Facebook isn’t (yet) is user attrition. The Guardian’s Arthur observes that one research firm had pegged the number of Twitter profiles created at around 383m at the beginning of the year, so the 140m figure Twitter is touting hints that the company’s service isn’t for everybody.

Even with all the question marks, Twitter’s growth, coupled with its age and the amount of funding it has received, raises the question: will we see a Twitter IPO in the next year or two? If Facebook’s IPO is a success and its share price doesn’t sink dramatically in its first six months, it wouldn’t be all that surprising to see some of Twitter’s investors pushing for a TWIT listing.