I haven't tended to friends-lock posts very much, but we're experimenting with Facebook at work (a lot of libraries are trying to figure out how to use social software to reach users, and Facebook is well adapted to this type of thing.) When I set up my profile I configured it to pull in my LJ posts as a Notes feed.

Anyway, I ran a little experiment and discovered that if I friends-lock an entry or set it to private, it won't show up in Facebook.

All of which simply means that if I get really mad about something and rant, I may friends-lock the entry so I won't seem like an unprofessional maniac on Facebook.