Cynthia Typaldos has created the list you've been waiting for (or at least she's started it and is waiting for you to finish it), namely the list of all (Yet Another) Social Networking Services with perceived focus. The list is already at over 2 dozen such sites. In addition to the version on her weblog, she's posted an online spreadsheet view of same. She also agrees with Marc Canter that these services are vulnerable to a more open strategy:

Unless >2 degrees of separation and node secrecy are valued by users (maybe not everyone but an interestingly large set of users), an "open" networking service will make these proprietary services and software obsolete before they've made a penny.

She's done us a service by making this list -- take a look and help her flesh out the details.