Does online life promote the expansion of one's mind, or rather its enclosure within definite parameters?
Love the recurring thought here that it's ultimately up to the individual. We have more tools for more investigation of more viewpoints, that's for sure. But we also have more tools for enclosing oneself within one's views, never really confronting the other, never really having to subject our thoughts to anything outside our bubble. I choose to focus on the first path as much as I can. Sometimes it's depressing to see so much of the second path being pursued by so many.
I believe the answer to your question is entirely dependent upon the mind of the individual itself. Certain minds do not crave challenge to their beliefs, and for them, spending time online will simply serve to re-enforce their pre-existing ideas about the world. Other minds crave challenges to their beliefs very strongly, and online life is one of the best ways for them to be exposed to the ideas that will do just that. So you have to ask yourself how rigorously you investigate the other side of your beliefs, if you wish to expand your mind.