I want to complain about Twitter, but I don't know how or where. It's been working worse and worse lately.

You just have to go to PissedConsumer. It's a website that allows consumers to leave reviews of products and services they have purchased, as well as complaints about the companies that have served them. The PissedConsumer site serves the following functions: Providing a platform for feedback, increasing market transparency, improving the quality of products and services, providing a platform for conflict resolution, collecting and analyzing feedback, and more.