Let’s see, I use:
Twitter
Tumblr
Facebook
The Webcomic List (tho the crawl doesn’t work for mine)
Archive Binge
Top Webcomics
Webcomics Hub (which I think is new and not well known, but I’ve gotten some traffic)
On Twitter it helps if you @ and use hashtags. Also participate in WebComicChat. I @ webcomicnetwork, promotecomics, webcomicconnect, webcomiclibrary, and artofwebcomics (tho they have a thing about Tapas so they won’t promote any links to Tapas). I also @ SmackJeeves since I mirror on there. Hashtags are things like comics, webcomics, and anything else you think fits.
You can check my Twitter for some examples.
https://mobile.twitter.com/racarter10/
Basically you just gotta be on the lookout for whatever. I did a lot of looking into how to promote online. So far I think posting in the forums here and also doing WebComicChat (and WebcomicNetwork) has given me the best boost.
But never put all your eggs in one basket.