I think this is your problem. All of the compelling virtual worlds I've been part of have some hook that works on the individual. If you doubt, log into one of the old classic MUDs like LegendMUD (http://www.legendmud.org/) or Blood Dusk (http://mud.dusk.org/). They're still compelling when you're alone in the world (and these days, that's pretty likely).
I think it's perfectly legitimate to say your product is better in groups. I would hope so for something calling itself an "MMO." I think it has to be compelling by yourself to some extent though. I'd argue that every major virtual world from WoW to Second Life to even Minecraft has provided this.
What's even better is that when you have that action that is compelling alone, the second person into your virtual world sees that person being awesome and then desires to join in. You open yourself up to getting a "First Follower" effect, in other words.