Fun comment, and Dropbox probably is winning by quite a bit, but Dropbox's referral system for free storage could be the explanation for the Twitter traffic.
I'm with you, although I do think the UI leaves something to be desired. I'd probably recommend DropBox to my parents -- at least until Windows makes "Live Mesh" just a built in part of Windows.
Question for you, in the old version of Live Mesh there was an Explorer extension, where I could right-click a folder and say, "Add to Mesh". It appears that this is gone and I have to go to the Live Mesh app to add a folder. Is there another way to do it?
One of the reasons (after unlimited p2p sync and 5GB cloud sync) was the ability to sync ANY regular folder on my HDD...
Turns out they didn't consider the UI as too confusing or complicated.