I organise all of our club runs. We used to use Facebook to let our members know about upcoming events. My biggest frustation with Facebook was when members responded to event invites with "maybe". This meant I never really knew if that member was coming or not. I generally assumed "maybe" meant "no", especially if it was raining, but it made it so hard to get a feel for attendance numbers.
When I first found I'm In, the first thing I spotted was the lack of a "maybe" button. At that point I knew I had to start using I'm In to organise our weekly runs.
The other issue with using Facebook was that not all of our members use Facebook, so we had to do some manual arranging still. The great thing about I'm In is that it is email based, and there is no sign up required for any of our members, so there's no need for anyone to remember their password!
Some of our members are not the most tech-savvy, however, everyone seems to manage responding to the invites just fine.