Safari extension to check if you're currently logged-in to Google/Facebook/etc.?
I don't know of an extension that will warn you about remaining logged into Facebook or Google accounts. However, since your goal is to block web trackers from services like these, a better solution is to install an extension that blocks web tracking bugs. I personally use and recommend Ghostery. Ghostery works with Safari, plus Chrome, Firefox and IE, and it blocks Facebook and Google trackers, plus a slew of others as well. With a plugin like this you can also safely remain logged into your Google and Facebook accounts as well.