Thanks to a tutorial from @swentel my attention got back to webmentions again and I gave the first release candidate for another try. What's still confusing to me that I have to enter a link-class and a publishing target manually. The former might be a task for a link widget and the latter may be some lack of understanding the API so far.
The Drupal looks pretty solid and feature rich. Webmentions - the new thing on the web - makes me hope that the dependencies on "social" networks like Twitter, Facebook, Reddit or others could be massively reduced if not eliminated. Sounds like another major step towards de-centralising the web. Thanks a lot to all the contributors to Webmentions and the Drupal integration.