Back in the old days the science fiction super genius Damon Knight wrote a story called "I See You". He's one of the giants of the field, and wrote some amazing classics (for example, "To Serve Man").I don't know if you can get his collections anymore, but you can get an ebook at the EBookMall, which has the summary:
A machine is invented that can view any person, place, or thing at any time in the past or present anywhere in the universe. The implications for politics, personal privacy, crime and punishment, space exploration, and historical research are mind-boggling and brilliantly explored in this short story. Hugo Award Nominee
While we don't quite have the "anywhere in the universe" part yet, we are nearly there as far as "the present" and even the recent past.
All that is needed is a user interface to an abstraction layer over all the traffic cams, surveillance cameras, nanny cams, cell phone videos, paparazzi, drone cams, flikr's, street-views, facebook's, you-tube's, etc on the internet. And your friendly search engine, of course. You could pick a place, say Trafalgar Square, and a time. The new tool would stitch together all the video and image sources to allow you to change viewpoints and pivot around the action in 3-D.
While this is fun, combine it with face recognition software to take the idea of "following" someone to a whole new level of creepy stalking. Now you are not just waiting for the next 140 characters but can actually watch them as they type. Set alerts to be notified when they emerge into view of any camera anywhere in the world. Back up the action and watch from a different angle. Use video interpolation software to fill in any missing parts.
Now go back and read the story. Just add a little more programming and we're there.