RV net posting volume ebbs and flows with the time of year, economy, price of fuel, weather, and a host of other things. There's also how many members have something to write about and then actually write then post a contributing thread. Lots of folks are only interested in reading topics they are interested in but would never dream of posting. Most everyone likes a Trip Report which is the first thing I check for myself to see who went where and gain a little information about it.
I think the forum format is good for what it is. Facebook alerts your friends of every little thing you write but most of the people I know via Facebook are not interested in RVing so there is no other contribution nor demand for information. Just not a good place to share the interest.
RV.NET is great in that way because we all share the interest. The user numbers will go up and down but because it is a specialty interest website, it may never have a huge following but it will have a consistent dedicated following.
I imagine the best way to increase numbers is to be sure to share the information about the website with those you meet along the way that would be interested.
RV.NET is in my opinion one of the best sites on the web.