FAQ 202 [http://www.livejournal.com/support/faqbrowse.bml?faqid=202&view=full] currently talks about finding people on LJ, but a frequently asked question that we get in Support is how to actually *contact* said people.
This could be its own FAQ, but it could dovetail with FAQ 202 as well.
Points to include, in super-rough bullet point format:
* LiveJournal does not currently have a user-to-user messaging system, although such a system is under development. [This would be its own FAQ later on, of course.]
In the meantime, you can contact users in a variety of ways:
* LJTalk is an instant messaging system that supports integration with LiveJournal. If a user lists instant messaging contact information in his or her profile, you may be able to contact that user via LJTalk. [linky to LJTalk FAQ http://www.livejournal.com/support/faqbrowse.bml?faqid=270&view=full]
* See the profile for other instant messaging services that a user may have listed.
* You also may be able to contact a user via email or text message, if the user has made such information available.
* You may be able to leave a comment to an entry in a user's journal. If a user has comment notification emails enabled, he or she can receive a copy of your comment via email. [linky to commenting FAQ http://www.livejournal.com/support/faqbrowse.bml?faqid=62&view=full]