IMHO there is little chance that an online-only community could replicate the successes (many friendships among the members, very high levels of enjoyment, motivation and engagement) of LW NYC.

Why not? Well, at the risk of putting off those readers who dislike explanations from evolutionary psychology, friendship relies on complex functional adaptations that were "tuned" or "designed" by natural selection for an environment in which every friendship has significant costs. By "costs" I mean that either the friends had to pay the social cost (which was significant in the ancestral environment) of being seen to be talking to each other or they had to go to significant trouble to talk without being observed. Even after the rise of the city (where unlike in the ancestral environment, most observers do not care who you talk to) maintaining a friendship had costs, in that the friends have to commit to being at a particular location at a particular time, incur transportation costs, etc.

My theory is that there is important information in whether (and how readily) a friend continues to choose to incur the costs of maintaining an off-line friendship and that when that source information is lost, most people have trouble accurately assessing the value of the relationship and start to make bad decisions on how much time and mental energy to invest in the relationship.

IMHO the same argument from evolutionary psychology holds to a lesser extent for the sense of belonging that people feel for various groups and communities. There was for example probably nothing like a lurker in any community before the online communities enabled by the BBS, the (now defunct) proprietary computer networks and the global internet.

Online-only communities and using the internet to keep up with friends can be extremely useful of course, but a person should watch out for the common failure mode in which online participation lulls one into a false sense of belonging or connectedness which prevents one from deriving the benefits people can get from things like NYC LW and the visiting fellow program in the Bay Area -- benefits that most people here should pursue and that are not available without face-to-face interaction.