Write to a friend
Clicking on the photo or name of a friend send you on his or her profile, where you have the Private message button to start a conversation.
Write to a member (not already friend)
Clicking on the photo or name of a member send you on his or her profile, where you have the Private message button to start a conversation.
You can find other members in Members menu item, in public Groups, in Areas of interests lists, etc.