I have not been able to find out how many people are currently on Empire Avenue (EA), nor any data as to the typical profile. Intuitively and from my experience, the early adopters are clearly all closely linked in one form or another to social media -- in other words, not necessarily representative of the broader population.
Re your second question, I see EA as a hybrid. Its success resides on being fun (game) and purposeful (business). As such, you will find a wider audience appeal, making the reasons for playing broader. Were it to become too businesslike, I believe the buzz would be taken out.
To that end, I have seen many participants that are only playing the game based on the investment opportunity (the pure game aspect) and who do not themselves have a social media presence to speak of. Between gamers, self-promoters, heavy bloggers, brands and EA enthusiasts, to my understanding, it is quite a mixed breed.
To answer your third question, I find that the networking is different from the other social media platforms in particular because of the different communities to which I belong, where the membership is different from the people I have been following or know in the other SM circles. I haven't yet transformed an EA acquantaince into a real-life meeting, but I have had a few unique exchanges that I am sure will one day convert.