Turns out: Twitter left my mind officially. These days I look into Twitter only on a randomly monthly basis. I just don't care much about all the retweets and promotional stuff. As I'm still cross posting to Twitter I enabled a setting which is supposed to send me an email for “notifications”. I thought this would cover the few cases someone writes me over there. I was wrong. There are now direct messages piling up and I had no idea about them.
First of all, sorry for everyone who had to wait for ages to get a reply.
Second: I'll change my behaviour now. I'll look into Twitter once a week to see mentions and direct messages. If you have something that feels more important, please send me an email or a webmention.