Now that OCR is bigger than triathlons, marathoning (!) etc. combined, you can leverage Facebook Ads targeted to that crowd.
They tend to be younger and willing to try new things. You're on the right track with FB ads and FB events. When one person signs up, they invite their friends, who humble brag their entry for social proof within their clique or community. OCR is way more popular than triathlon now, but younger "one and done" triathletes disillusioned by the douchebaggieness and great financial/opportunity COST of triathlon are looking for something fun, more social. Right now OCR has that market, 2+ million last year, but most of those folks would give AR a look if the races were very short and newby-friendly.
The paradigm for success will always be the HiTec series of the late 90s, early 2000s that REGULARLY had 200, up to 300 teams. These were shorter races, 2-3 hrs for the podium teams.
If you look back, these were a hybrid of AR and OCR. This formula has ALWAYS worked! This is what newbies AND fast people want. A race course that is fun and interesting.
These race combined sprint AR with OCR. There were mud pits, 10' climbing walls, etc. COPY THIS FORMAT. Advertise using FB targeting OCR, triathletes (beginners) , mtbers (who can all hike)
The 1998 race:
http://articles.latimes.com/keyword/adventure-raci...
"Nearly 1,000 racers…"special tests"…
http://articles.courant.com/2002-09-08/news/020908...
"1,000 racers…demographic…"
http://articles.courant.com/2002-09-08/news/020908...
A FB ad would need to depict a team of 3 Riding mtb; paddling their inflatable ( cost ) and running with map and compass. You have to show all 3 in your ad to differentiate, since Competitor Magazine, Outside Magazine, Runners World, etc. all decided to call color runs, mud runs and OCR "Adventure Racing" which infuriates me.
A one second glimpse at the ad should give the audience an idea of what the event entails: team, paddle/run/bike, navigation. A look at the race website should show that the team (they'll perceive it as a multisport "amazing race") will have "to use their wits to decide whether to swim across the lake or run around" type navigation.
This month's Mud & Obstacle Magazine has a several page long article on Adventure Racing, and how it's different than OCR. Thank you, Kristin Horowitz! Kristin and Yishai put on a 12hr event in Central/Southern California that is well-attended. They also put on OCRs and also mud runs I believe. Reach out to them on FB or their website
http://all-outevents.com/about-us-all-out-events/
You're never going to get a large number of newbies into 8-12-24hr+ AR at once, especially now when there isn't any significant prize money to attract fast OCR/XTERRA types.
AND FOR GOD'S SAKE…no optional CP bullshit. That destroys the whole reason they're doing it in the first place…to share their STORY, their experience of traversing the course, making decisions, etc. Optional CP/rogaine syle ARs have contributed to killing AR and are for lazy RDs. Save the lame excuses about putting on an AR in a small state park. Call it something else, like "multisport rogaine" but not AR.
The market for the longer stuff will always be 95% smaller. I competed in 24-48-100hr events in the 90s and early 2000s on teams of 3 to 5, and even then it was an "older" crowd for the most part.
My 2 cents.