The idea I like best out of what has been suggested is to make the "other site identities" field searchable. That way you have complete control over whether you want to populate those fields, and which online identities you want to link up or keep separate. Would need to be expanded to include LJ.

I don't really want to be findable by email address, even if the security issues of address-book scraping can be addressed. My primary email addresses are linked to my birth-certificate name, and I use them for business interactions. For this reason, I use throwaway addresses to join social network sites anyway.

I'm quite wary of even notifications that someone has joined DW; right now Google+ is cheerfully telling me exactly which of my rabbis and bosses has signed up for the service. Even if they have been very careful to keep their profiles sanitized, this information is in itself is a privacy leak. The other thing that this system leaks is the social graph; I really don't think it's appropriate for every customer service grunt I've ever emailed to have a way of finding out whom I'm connected to online!