One of the things I want in a social blogging substitute is the ability to point to posts. So instead of n% of people linking to an xkcd feed, you can click a button that says "share this", and then that post appears in your friends' friends pages as by xkcd, tagged by you. You'd have to exercise restraint that you didn't just tag everything cool (reddit is for that), but for "this is cool, everyone should read it, but, you know, only once"... :)
Yes, but like baljemmett said, easy to drop into an existing system. I seem to work best with a primary social site based on writing things. Although I guess it's only a problem with lj (and similar) because lj has friend groups which *overlap*, with usenet or message boards, if someone posted a link, it's right there.
Hmm, interesting idea. Facebook has buttons labelled "share this" on thing likes photos and "notes" (posts), but I've never investigated what they do -- I'd guess they either plop a message in your inbox, or add a "Johnny Random shared Joe Q. Stranger's note with you" line to your news feed. The latter would be in line with what you're suggesting, I think.
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