The vast majority of activity for Na’vi is on the learnnavi.org website, especially the forums. The community, similar to Klingon’s current main forum, is about 80% monolingual fandom and 20% fandom that has invested time into learning the language.
The math here is critical to pay attention too. What happened was two conlangs were associated with two huge fandoms. A sliver of those people decide to learn the conlang. I would have predicted that a language without such a overt conculture would be more successful on account of culture-shock effects.
FANS * .0001 – CULTURE SHOCK * .5 = CONLANG USERS
So if the # of fans is huge, the loss due to culture shock is outweighed by the great mobs of fandom. (I’m picking .0001 and .5 as some arbitrary small numbers)
A conlang with a conculture and no pre-existing fandom will look like this:
0 * .0001 – 1 * .5 + 1= ~ 1 Conlang Users
The constant is for the inventor.
If we forget about fandom, and use an equation that should work for French as well as toki pona, we get:
EXISTING COMMUNITY * .5 + PROMISES * .5 – CULTURE SHOCK *.5 = # USERS
So if a conlang designer wants fans, you can hijack an existing fandom the way, say, the Vulcan language designers did, make a lot of promises, do something to minimize culture shock and build your community. Highjacking an existing copyrighted IP, like Harry Potter or Star Wars, is kind of risky. So that really only leaves everything but fandom based strategies.