After updating it (i.e. the linux app/program and the extension) today, it finally works. On twitter and on facebook (and probably more websites) a keybase chat button appears, and clicking it a small chat window successfully transmitted a message via my local linux app. My expectation to be able to see whether someone uses keybase.io or not, was not fulfilled - this information is accessible only after clicking the chat button (or the symbol in the browser tool row).