Every politician nowadays has a Facebook page.
I've just left comments at the pages for both Udall and Bennett. Would really love to see a flood of posts supporting the RKBA on those sites.
Mark Udall's page is open enough that you can post directly to his wall, which means any post is immediately and publicly viewable.
https://www.facebook.com/markudall
Michael Bennet's page does not allow you to post to the wall, but you can respond to the things that he's posted.
https://www.facebook.com/senatorbennet
If you post, be polite, don't make gun owners look like troglodytes.