Once you add friends you can go to the top of the page and select "Quick Links" The select "Open Contacts Popup"
This will popup a box of all your friends. It will show you which ones are online and Which ones aren't. You can also select multiple friends and select "Send PM" This will let you send a pm to all your friends that you put a check mark by.
Say you want to send a PM to all your friends that are online. Check them, then select "send PM".. It will automatically place them all in the Too: line and you didn't have to type them all out...
Or, if you want to check if someone is online so you can chat via PM.. Check that contacts popup and you'll know.