Thanks for the reply - it looks like it was an intermittent glitch.
I'm a web developer, so have some background debugging similar issues.
What I was seeing was when I loaded an article in a new tab in Chrome, it would load correctly, but then would redirect to a Facebook URL with the MMM article URL as one of the parameters, as if it was trying to integrate some plugin or feed from that site. It would then get an 'Authentication failed' error message and stay on the Facebook page.
Seems to be working okay again now anyway.
I also got a random CMS database looking error message when I was trying to reply to your post earlier - I must be cursed today :) Screenshot attached: