Ruby's biggest boost of popularity occurred between 2006 and 2008. It has tailed off a little lately as it's found most of its audience already. A lot of people jumped from languages like Perl and Python (and the less determined Java folks!) during that time.
I don't really like to admit it, but Ruby on Rails played a big part in getting people interested in Ruby. Before then, it was still a cool language but it was very Japan focused and there weren't many good libraries for it. Rails provided the impetus for people to get on board and start developing a lot of non-Rails stuff too. Ruby now has a library set as rich as for any other language.
Author of Beginning Ruby (Apress): http://bit.ly/t31ag Editor of Ruby Inside and RubyFlow