There is an option on each profile to show an "is online" button to friends, everyone, or no one. I have mine set to "no one" for a couple reasons. I rather not have everyone know if I am online or not and also the "is online" may stay on if I am idle (am logged in, have a medhelp window or tab open, but not doing anything at the time on the site.) I've heard that causes confusion.
We do not currently show who is online when you click See All for friends, but this feature is available if you look at a specific user's profile. Just click on your friend's user name and if they allow others to see their online status, it will be displayed in their About Me section.