I think it's a really bad idea to start using Twitter followers as a credential. That being said Phil Plait is a legit astronomer and has a Ph.D in Astronomy and worked on the Hubble Telescope and has done a TED talk on defending Earth from asteroids. With real credentials like those you're doing a disservice to first mention the number of Twitter followers to validate his expertise.
I'm pretty sure mentioning the amount of twitter followers he has as credentials was a joke, but I agree with your point. I don't think anyone would contend the fact that Ashton Kutcher had the final say in all matters.