Trying not to repeat points already made by others, I'll tell you this:

The simple act of reading and working with other people's code is a great thing to improve. If done properly, of course. Going on your own, specially when starting, is how you end up hitting walls.

Sure, having someone act as mentor explicitly is indeed a great help, but even without one, simply collaborating with others will also help you a lot. You just need to approach it correctly. Listen to others, to their motives, their reasons for doing things in a certain way. Listen to that even when it is about doing things in a bad way or in a way you think is bad. As an example, it's not that important that they are using Backbone; the important point is why (and to a point how) they are using it. You can learn much from the whys and the hows of other people.

Clearly you need to find a team that is open to this behaviour. So, when interviewing, be curious to get a good impression of the team and their general attitude and organization.

Also, notice that, whatever you do, simply by staying put you're already favouring stagnation. If you can find a path of progress in your current job, anything else will be better (to combat stagnation; there may be other factors for/against changing jobs, of course).

Others have already mentioned OS or pet projects, which are also good, so I won't mention them. I've found "meet ups" or groups to be somewhat hit or miss. YMMV but if, as you mention, you find it hard to socialize it will probably be hard to get something out of it. Maybe try some smaller group or one that is just starting so everybody is a stranger to everybody else.