What's the best name for a service object in Rails?
You could use modules to group them which would potentially shorten the names too. You'd probably need to look at the types of service methods you have before deciding how to group them (e.g. by model: Runner::MembershipStatus or activity: Statistics::RunnersWithGoals).

Presumably you've already decided you don't want these methods on your models? For me a service method makes most sense if it's performing an action rather than just retrieving some information. So GetCurrentGoalOfRunner would probably sit on my Runner model as runner.current_goal rather than being a separate service. 

Possibly also worth looking at Mutations (which I was introduced to recently): https://eng.uservoice.com/blog/2013/02/27/introducing-mutations-putting-soa-on-rails-for-security-and-maintainability/