Using someones internet connection?

[Lateralus]

not only can they see what you're visiting, so can anyone within range of the signal... if you're using a wireless connection without encryption and a password, anyone around can see everything that goes on (though some websites will use encryption of their own).

If your webcam is already activated and running for a different reason across the internet, then it's possible, they can see it like everything else... but they can't activate it at all.