Well, that's a bit difficult. Mostly because I tend to focus on the moments of the source material and how they affect me. What fandom does can sometimes be the subject of unwanted attention (witness some of Bronies, all of Puppies, the entire GamerGate splat), but then there are also things that are heroic and awesome that fandom does, like the Harry Potter Alliance. And the charitable donations in the name of fandom.
I think that the biggest impact that fandom does, or has done, are things like the 501st Legion / Rebel Alliance and similar cosplay groups that make it so that characters from fandom can spring into life and allow others to experience heroes. (Even better when you can get the actors to get engaged, too, as with the Iron Man prosthetic arm.)
Which points to the thing I find most touching of fandom activity recently - Batkid. The Make-A-Wish Foundation does awesome things for kids and funds then through donations and volunteers and asks people to get on board and provide time, expertise, and money. A young child's wish was to be a superhero.
San Francisco (and a lot of the surrounding area) responded. In the thousands. And the tens of thousands. So much so that the Foundation had to say "Thank you for your interest, but we have enough people to help with this." And all the fans turned out in droves, as bystanders, as other heroes, as members of a grateful city, as journalists and members of the media, as the entities of justice that help and handle the matters of prosecution. Even the President got into the act for a little bit.
For a day, fandom became the very best that people could be, to make a child's wish come true. Sure, the scenario was scripted, but the response and the enthusiasm that everyone brought wasn't. I'd love to see more Batkid moments in the future.