There kinda is already, sorta. On your friends list (on the left) you'll see 3 horizontal bars. One over the name, the average and the light columns. If you click on the bar, it will sort that column. Click it again to reverse the sort.
Unfortunately it's not very useful right now since, as you mentioned, we can only have 10 friends at a time and those are all it sorts. :( You're also correct about it being the first 10 added. Hopefully the new year brings some new fixes too. :)