The outpouring of earthquake-related tweets suggests East Coasters seem to mostly be enjoying the uniquely unsettling feeling of "who the heck is driving a big rig down my street OMG IT'S AN F-IN EARTHQUAKE." The reaction is equally amusing to people who live on the West Coast, where we have real earthquakes thank-you-very-much, and this regional geologic snobbery is predictably also in evidence on Twitter (apparently Californians don't think Mother Nature is on Twitter, or they might hush up with the bragging -- also, we're boasting about something that lowers our property values?).
But in the end there really isn't much to say about a small-ish earthquake (and thank goodness for that). On Twitter you basically get the same kind of exchanges you'd have in person or over the phone in a previous era: "OMG did you feel the earthquake? I felt the earthquake!" "Yes, my dog felt the earthquake too! He was all, like, 'bark!'" Then of course there are the sad folks who somehow missed the earthquake, but they can still tweet about it: "I totally didn't feel the earthquake! But my cat was acting weird!"
That's not to downplay the damage (apparently some property was destroyed near the epicenter in Virginia; hopefully no real injuries) or the alarm felt by people during the experience itself -- when it's not clear whether you're feeling an earthquake or something else which could be much worse. In fact, the relative rarity of earthquakes on the East Coast probably makes it even scarier, since it might not be the first thing that jumps to mind when the ground starts shaking. There is also an old nuclear plant about 20 miles from the epicenter in Virginia, so it's not like fears of a "real" disaster were that unreasonable -- but I hasten to add that the Dominion Virginia utility company says they shut down the reactors without incident.
On that note, I think a lot of the self-expression on Twitter is psychological venting, as people joke about the experience to share their feeling of relief. One hilarious mini-meme which emerged on Twitter almost immediately mocked cooler-than-thou hipsters boasting "I felt the earthquake last week" or casually mentioning "yeah there was an earthquake but it was underground in Williamsburg, you probably never heard of it." Then there's this great photo (http://jmckinley.posterous.com/dc-earthquake-devastation ) of "earthquake devastation in D.C."
But social media isn't just for joking around. Even following a minor earthquake, social media sites like Twitter can play a valuable role in disseminating information and allowing people to connect, despite communications disruptions. For example, it appears that cell phone service and possibly even landlines are down in several big east coast cities, including New York and Washington, D.C. Part of this is probably due to everyone jumping on their cell phone to ask "OMG did you feel it?" but it could also be due to actual infrastructure damage. Either way, sites like Twitter are clearly picking up the slack in allowing people to stay connected with each other. By the same token, I'm going to guess email volume spiked, and I wouldn't be surprised if 24-hour cable news ratings shot way up, for at least a little bit.
Of course there's a marketing angle too -- no doubt someone is going to jump on Twitter (or has already) to try to ride the #earthquake trend. Off the top of my head, maybe something like, "That #earthquake you felt was from people racing to our store to take advantage of our 30% sale on designer shoes!" Normally I'd advise against this in a natural disaster, but since this wasn't the "big one," it might actually be effective (if done with humor).