Today is Lady Gaga’s 25th birthday, and what better way to celebrate than being the first person to reach 9 million followers on Twitter? Her fans have responded by trending the #bornthisday hashtag for the past 24 hours.

Good times. Unfortunately, these happy days probably aren’t going to last forever, as Gaga’s place as Twitter’s most-popular user is under threat from Canadian pop star (and all-around social media irritant) Justin Bieber, who is now just half a million fans behind.

In the last 3 months, Bieber has added over two million followers, compared to about 1.5 million for Gaga.

Their follower curves look similar, but Bieber is definitely catching up, and his speed is increasing. And the bottom line is his fan base is simply a lot more rabid, and might even see this as some kind of revenge.

My prediction (and much as it pains me) – Justin Bieber will be the first person to reach 10 million followers and will be Twitter’s numero uno by the end of May. At the latest.

Last month, Gaga lost her place on Facebook as the most-liked real person to Eminem. And there are still other threats on Twitter that have yet to show their cards.

Still, I think it’s fair to say that, right now at least, she’s probably the most popular celebrity on the planet, but judging by these events one has to wonder for how long. After all, the same rules apply to her as to everybody else.