So 4 players in a wing is not MMO but 6 players is? Sorry that doesn't make sense to me.
I have been in CGs cooperating live with 100's of people (across many groups and instances) and had dozens of pirates in groups attempting to stop us - that is a massive social interaction - far more than you get between different groups in most MMOs. In very few MMOs do players (grouped or solo) have any effect on the game worlds in anyway at all. Effect on the Game World or on other Players is not a definition of an MMO.
In ED You are playing online in a game universe that has potentially 32 players in a single instance (or more if you are far enough away from npcs it seems) and, as far as I can tell, potentially up to 3/4 million players at one time in the galaxy that you can (with a little jiggling

) meet up with.
However rarely in other MMOs do you see more than 32 players in a single instance (or visible at one time) and apart from Eve and ED (afaik), no other MMOs out there have more than a couple of thousand players max on a single server (ie the number of different individuals you can potentially meet in your avatar's existence is a 500-1000 times less than in ED/Eve. Although I have played in MMOs with larger numbers of people visible (on multi-group raids for example) less than 32 players (or even 16) is the norm. Once you are placed on that server world, you are then instanced to keep numbers within manageable limits and instanced further when you are in dungeons/levels (either solo or grouped).
For example SWTOR. You are seperated in to servers, You have no more than 6 in a group. You rarely see more than 16 players at one time. The game pulls people you group with into your instance. You have no effect on the game world you are in at all. 1/2 the level/dungeon areas are instanced to the group. - that is an MMO though isn't it?
Just because ED is different from many other MMOs (that people are used to) it is still an MMO. (IMO

)