Logout of Google and Facebook SSO

I use a discourse forum for coworkers to discuss work related topics. When they login from their work computers, it also logs them in to gmail or facebook, which some are not aware of. Then when they log out of discourse, their gmail and facebook remains logged in. We haven’t had any problems but I can see this could be a privacy issue as we have more users using it on public computers.

Is there a solution I can implement such as automatically logging them out of Facebook or Gmail?

Unlikely as that login control is handled by Facebook / Google respectively.