This feature is premature optimization. It's at its most useful on a site like Facebook, which is primarily about social networking and has millions and millions of users. It's at its least useful on a site like Dreamwidth, which is about meeting and connecting with interesting people / communities and has only a handful of users. If someone you already know is here, she either came with you or probably doesn't want to be found.

Not only are the kind of people who come here more likely to be hostile to this suggestion (as some of the comments show), but the kind who would love it wouldn't get much use out of it. It's hard to get right, doesn't have an established standard to play off of, adds another feature to a feature-rich and straightforwardness-lacking environment, and will have a negligible effect if it's done well.

I think helping people find readers and new things to read would be more beneficial, to Dreamwidth's community and its stated goals.