Whoa! I'd been up at 5am and everything was working fine, came back to the computer at 8:30 and Twitter was down, LiveJournal was down, and FaceBook was creaky, barely holding on.
WHAT THE FUCK WAS GOING ON???
At least I got one interesting datum out of the Very Irritating Situation ... the guys over at LibraryThing.com pointed me to http://downforeveryoneorjustme.com/ ... a thing that lets you check if a site is actually down, as opposed to your having issues reaching it. Very useful. Didn't help with my frustrations, though.