Official comment
Hi Matthew - the SSO feature is on our roadmap but development work has not yet started, so I can't give an ETA. It will allow social logins using Facebook, Google or Microsoft credentials. Configuration options will allow you to enable them (or not). It is high on the list of features to be added.
One thing that might help with your current membership sites: We recently added the ability to adjust the member's "automatic logout" setting so you can control how quickly they are logged out from the membership site due to inactivity. You can adjust this in Pages > Membership by opening the membership site configuration and adjusting the value. The default is 24 hours but you can choose to never log people out, or set shorter or longer times. Here's a screenshot of where to find that feature.