Are you required to respond to/thank someone who follows you on Twitter?

[Bec]

I wouldn't bother unless you wanted to.
I mean celebs who have 25 thousand followers don't go and thank every single one. They might post once a year saying "thanks you guys for sticking with me and following me" - but they might not.
Sorry to burst your OMG! bubble but to him you are 1 of however many followers - he's probably not going to notice your tweet thanking him (he might do though but he probably wont respond to it).
I get the omg moment though - I like a page on facebook that sometimes has questions with a band - first 30 questions submitted get an answer. And when you see your question in the top 30 you have a moment to freak out. Then the band read and answer (using your fb name to address you) but then you come back to earth and realise that to them you are just another fan who they don't know