It has been known that webcams are easy to hack and any privacy conscious individual should put a piece over the webcam build into their laptops.
Criminals have been known to hack into webcams etc.
But now we know that the government has been watching the world's webcams on a massve scale. According to a report in The Guardian based on documents leaked by Edward Snowden, The UK GCHQ has been monitoring Yahoo on a massive scale. In one 6 month period they took/obtained pictures of 1.8 million people. None of this was targetted at criminals or potential terrorists. They simply spied on everyone.
While this was on Yahoo and between 2008 and 2010, don't expect it to be limited to that timeframe or that network