More by chance I noticed that I am following a twitter account called something like Greyson Chance fanclub Germany, which I didn't (knowingly) follow before. It turns out that the account is older than the Youtube post by the guy, so this cannot be a account that was legitimately created. The account contains one post via API from February, so I probably followed the account under some other name since it was not obvious spam and the account was later sold and renamed.
This would explain the frequent accounts that have no or only one post but are following everybody.
(Apr 23 2010)
(Apr 28 2010)