I think that there is much more 'value' in an upvote or a downvote of an article / comment than just the 'karma' the way HN,/., digg and so on use it.
Easier said than done though :)
Also, I absolutely suck at design...
Otherwise you can aggregate bunch of social network profiles already, and for the ones not supported you can add a custom RSS feed.
I have a beta management site (http://www.launchset.com) that could be useful for your site. I'll drop you an email.
Thanks.
Besides that can any buddy suggest me any desktop application for such <- avoid suggesting flock coz i don't like it.
For friendfeed, twitter, facebook, flickr you can use feedalizr (http://www.feedalizr.com/).
Some people seem to like Seesmic for FB,Twitter if you're a power user.
If I could design a service... It would pick the _type_ of new stuff of each network of my choosing and aggregate like a Feedreader. With type I mean for example: new entries on my wall in a given Ning community, but no personal messages and aggregate those together with friend requests on Facebook.