This. Ashton Kutcher and Demi Moore were amongst the first celebrities on Twitter to actually post the content themselves rather than by a manager/PR agent. This got a lot of media attention and many other celebs followed shortly after.

I still remember finding it strange to see how Ashton and Demi personally wrote short messages, since every form of communication from famous people at the time were formal statements released via the press.

That happened a couple of years after it initially took off. Celebrities only took to it after it had gained solid traction among Silicon Valley types and social media early adopters.