Your experience: 

You don’t have to sign up with another form with all your information. You don’t need to remember another password. We’ll get your name, email address, birthday, and gender from Facebook. Eventually we’ll show you if you have any mutual friends with anyone in plans. 

We don’t get any other information from your Facebook account (e.g. your “Likes”). While we don’t plan on getting anymore information from Facebook, we’ll definitely let you know if that ever changes (also, you’ll have to give Facebook your permission to do so).

Bots: Facebook does a decent job with controlling bots. We won’t have to spend as much time and effort worrying about bots.

We will never post anything on your Facebook.

For us:

We have a team of two right now. Only one person is developing the software. Using Facebook login allows us to not have to build or implement our own authentication service. There is no way we can spend as much time or development effort as Facebook as on their authentication service (there's not that many software companies that can). 

Facebook login lets us focus on Coop and connecting people rather than spending time creating and maintaining authentication on Coop.