I have done a couple of things.
Started some Facebook groups for particular family lines. In them I post interesting historical snippets I have come across and links to their WikiTree prifiles for more info. I went through my mailbox and emailed everyone who would be connected with the group/s.
Hang about in various Google+ Communities (or newsgroups, Facebook groups, etc) where you might have an interest. Post interesting stories, and ask questions. If someone is having a problem with another site, it never hurts to mention the totally free and collaborative WikiTree as an alternative!
Spread the word to your local library. I have slowly been making inroads into libraries in my area. Until I contacted them they all had links to Ancestry in the Genealogy pages on their websites, but did not know sufficient (or anything at all) about WikiTree. Talk to someone in charge and tell them about the benefits. (If you are interested or can help do this contact me for more advice). I would like all libraries to have WikiTree among their links. Most libraries are essentially free services, yet the only genealogy links are to paid sites!