Twitter beat Facebook and Google+ as the fastest-growing social network in 2012, suggests a new study from GlobalWebIndex.

Overall, Twitter is now ranked fourth amongst all social networking sites in total active users, with Google+ securing a surprising second place.

Twitter’s 40 percent jump in active users between Q2 and Q4 of last year placed the micro-blogging platform well ahead of Facebook (+33 percent) and Google+ (+27 percent).

More than one-fifth (21 percent) of the global internet population now actively use Twitter on a monthly basis, good enough for a third-place tie with YouTube, but well behind Facebook (51 percent) and, surprisingly, Google+, which now boasts a 27 percent active user share.

Interesting stuff, but a couple of quick notes. One, the study has Twitter at 288 million active users, which is quite a bit above Twitter’s last official release (which was just a few weeks ago). And, two, until proven otherwise (ideally by Google letting us know exactly what constitutes an active user of Google+), the usual hefty-pinch-of-salt disclaimer for Google+ user data still stands.

(Source: GlobalWebIndex.)