Of course, if your peers are on different LANs but in close physical proximity then physical media becomes much more useful.
I think tribler defines "social" as those who you have social ties to rather than those who you are geographically close to. It thus leverages "socialness" for security rather than speed.
I think the tribbler's definition is correct. In the modern era, we often don't have social ties to those geographically close to us. Duality should simply be called a "geo-aware" client - not a bad thing.
For lightweight, on my router I have a transmission package (.opkg[2]) that handles files not available on my small network.
[2]http://en.wikipedia.org/wiki/Opkg
EDIT: Running Openwrt (Backfire, 10.03) on Asus RT-N16
What? Most of the things I torrent these days, I torrent specifically because I can download faster than any FTP/HTTP site will ever let me download.
Unless by friends you mean the extended circle of people that live nearby..?