You can choose to update your Facebook Wall through the Facebook Connect
feature any time you post a public entry to LiveJournal using the Post an Entry
page, or any time you post an unscreened comment to any public entry.
You can enable or disable Facebook or Twitter cross-posting using the Facebook Connect
option on the Extensions tab
of the My Account Settings page, as shown below:
The update to your Facebook Wall will say "[Your Name] posted a new entry to 'exampleusername' at LiveJournal." This notification will contain the subject and text of the entry you posted, and will link to the entry. A similar notification will appear if you post to a community. Any entries cross-posted to Twitter will contain either the subject of the post or a snippet of the entry, along with a short URL to the full entry on LiveJournal.
For comments, the Facebook update will say "[Your name] posted a new comment to 'exampleusername' at LiveJournal.", and will contain the subject and text of the comment, as well as a link to the entry. The Twitter update will contain either the subject of the comment or a snippet of the comment itself, as well as a short URL directly to the comment on LiveJournal.
Please note that the cross-posting options will automatically be unchecked when commenting to a Friends Only or Private post, so that no protected comments are sent to Facebook or Twitter by accident. Additionally, it is not possible to cross-post comments on protected entries or screened comments to Facebook or Twitter.