As a joke - let me emphasise: AS A JOKE (we'll get to this in a minute) - I seriously considered (aware of the irony) registering stalk.ly as a domain name with Libyan Spider and using it to host a site that billed itself as an online private investigation agency with the tagline, "We'll find out what you need to know... for a price." The whole thing was overall going to look as shady as reasonably possible.
You'd be able to find out exactly what you'd been up to and where you'd been based on your social media posts, with a timeline, map view, etc. There'd be a sign-up option that would offer you the ability to do the same for your "friends".
Worried your partner has been having an affair? No problem: slap their profile URL in here and we'll tell you exactly what they've been up to and where they've been up to it, etc. Want to know what your kids or employees have been up to? We've got that covered too. Oh yeah.
The point was to highlight how much oversharing most of us do online, and how unaware we all often are of how much information about our lives we're really giving away. Also I was more than a bit curious about how many people would actually try to sign up, and I thought it might be funny.
Fortunately (I think, although I still wonder sometimes) my tendency to laziness kicked in and I never actually implemented it. I was also a bit concerned about the possibility of unintended consequences that could be personally detrimental. I have no doubt Facebook et all would have gone ber-effin-zerk once they'd spotted the app, but that wasn't my major concern - I was more worried about the reaction of friends, and my employer, if by some chance it had gained some traction.
The ideal outcome would have been for it to provide a certain amount of entertainment for a while, and possibly drawn some small attention to issues of privacy that - prior to that point - had very much been minority concerns. Still, I was quite worried about it backfiring.