All kinds of stuff. There are lots of people I don’t know IRL who I’ve done projects with – writing projects, website projects, gaming stuff, other students… I haven’t met a single other student from my course, and until we graduate I probably never will, but I could envision a Beeminder goal to keep us working on a shared project.
If the question was “would you join a Beeminder goal with a total stranger”, that’d be different, but you don’t have to know someone IRL to know someone online well enough to have a shared endeavour.