Last month I wrote about how Justin Bieber had become the second Twitter user to reach 9 million followers, with Lady Gaga being the first.

I speculated on which of them would be the first to reach 10 million. Gaga had the lead, but Bieber was closing in fast. I suggested Gaga might hit the mark around May 23-24, but she’s beaten me – and Bieber – to the punch, as she’s now officially the first and founding member of Twitter’s 10 Million Club.

The milestone was reached about midnight ET, and Gaga celebrated in her usual style.

It’s another big milestone for Twitter, too. Ashton Kutcher was the first user to reach a million followers (way back in April, 2009). Charlie Sheen was the fastest, and Beyonce got there without writing even a single tweet. But being the first to 10 million puts Gaga in a different class entirely. Gaga has over 34 million fans on Facebook, but the network there is well over three times the size of Twitter’s, so like-for-like she’s on a par. Bottom line: she’s the biggest and most popular celebrity on the planet.

At least for the moment. Bieber is still catching up fast, as this chart from Twitter Counter shows.

I’d certainly expect to him to secure the number one spot later this year. Whether he can hold on to it is a different matter entirely.

(Chart credit: Twitter Counter.)