To be honest, roll calls don't really work for river cruises. First, when excursions are included there isn't really a need for a roll call to find people to share excursions with. (It is also difficult to coordinate your own excursions because timings of river cruise boats are quite fluid.) Second, when there are fewer than 200 passengers on a boat (and frequently considerably fewer) the chances of finding someone on your particular sailing are slim.
In general, most entries in the river cruising roll calls are for specific routes rather than a particular sailing. Sometimes they are split by company, sometimes the route and its ports are common to a number of companies. They discuss the excursion choices, and any optional excursions, and people weigh in with suggestions.
That said, much of the discussion goes on in the regular river cruising forum.
https://boards.cruisecritic.com/forum/115-river-cruising/