I imagine that Bungie will at least have in-game notices to go check out Bungie.net, like they did in all of their other games. That alone brought in a bunch of members who stuck around for the forums. I'm honestly expecting more than just in-game self advertising though, considering all of this talk about Destiny being "social at its core". I wouldn't worry about a lack of new members. From the sounds of it, it is going to get incredibly crazy and busy around here soon.