Tricky things, notifications. Overdo it, and you get bacn (not quite spam, not quite ham). But undercook it, and it's impossible to sustain online discussions because people don't know there are messages waiting for them. Consequently, I'm please to report that CiteULike has just (officially) implemented for:username tags, which can be used to alert users with whom you share a connection that something interesting is waiting for them. Users still need to visit the site to be notified, and won't get hassled by email (unless you send them one). In contrast, delicious's new share features veers dangerously close to bacn. Kudos to thegoose.
In addition, CiteULIke now also has "private" tags, though I haven't quite figured out what you'd use those for yet (cue for Fergus to leave a comment explaining their function):-