Why Are There Two Lists on Facebook Chat?by Micah McDunnigan
Facebook used to make you visit someone's profile to send him a message. However, its introduction of the chat service made it more like an instant messaging service with all your contacts in a single list -- or, in Facebook's case, two lists. The top list is for people Facebook thinks you're most likely to chat with, and the bottom list is for the rest of your friends.
You can have hundreds, or even thousands, of friends on Facebook. However, chances are you don't keep in regular contact with all those friends. The people who appear in the top of the Facebook chat sidebar are the people you've recently talked to or regularly interact with. This is a convenience feature to keep the people Facebook thinks you most likely want to chat with easily accessible at a glance.
The lower part of the Facebook chat sidebar, under the label "More Friends," lists your other Facebook friends who happen to be online. While the top list will display friends whether they're online or not, the bottom list only displays friends who are currently online and available to chat.
Who Is In The Top List
You can't manually set which of your friends show up in the top list in your Facebook chat bar. Facebook uses data from your sessions to figure out who you've chatted with lately and who you chat with on a regular basis. It then puts those friends in your top list. Facebook will add people to this list as you talk to different friends, and remove people from the list if you don't talk with them for a while.
People Not On The List
If you want to send a chat message to someone who isn't visible in either list you can use the search field at the bottom of the chat sidebar. Typing the friend's name into that field will make his name appear in the sidebar, so you can click the name to open a chat window with him. You can also use the search field as a shortcut to find a specific friend from the much longer bottom list.