It seems Justin Bieber is proving fairly popular on photo-sharing service Instagram. He joined last July, and has since collected more than 1m followers – the first Instagram user to reach that milestone. The startup recently announced that it has 27m registered users, meaning 3.7% of them are following the Biebster. In fact, the three most popular Instagram users are all musicians, with Bieber ahead of Selena Gomez (950k followers) and Taylor Swift (817k). Bieber traditionally competes with Lady Gaga for top dog status on social services, but unlike Twitter, Facebook and YouTube, Gaga has no presence (yet) on Instagram.