Everyone should watermark all images they place on the internet.... professionals and non-pros :) If your images cannot be traced back to you, they can become an 'orphan work' which means no known owner. In the future, orphan works MAY be used by a company for an advert etc. (even low res images on the net can be used!!) It's possible it might be used for an advert (eg a political party / arms manufacturer etc) and you totally disagree with the morals of that campaign!
Also make sure you resize images so they are 72dpi (I tend to make the largest side 7 inches max) and save files with your name in the files name (eg Helz Cuppleditch Zoo 2013). This file data is stripped and replaced when you place an image on Facebook, so all the more reason to add a watermark.
To keep up to date with the news about the UK Government's Enterprise & Regulatory Reform Act, visit the Association of Illustrators campaigning page on a regular basis.