Excellent point.
Gladwell's thesis is that high-risk activism and revolution are "strong-tie" phenomena and require real friendships.
He contrasts that with the "weak-tie" social media scene where people are friends on Twitter or Facebook, but wouldn't really risk their necks based on what their "weak-tie" friends tell them.
The "Save the Darfur coalition" Facebook page demonstrates what a weak-tie network can do and Gladwell's essay also talks about the (limited) usefulness of weak-tie networks (when he uses Shirky's example). It doesn't refute Gladwell's point
"He’s wrong. Facebook does motivate people to do things they wouldn’t otherwise do -– like donate to the Save the Darfur coalition via a Facebook page."
He agrees with the post author, that's exactly what social media does. But what it doesn't do is motivate people to go to Darfur, take a real risk and make a difference on the ground. That's his point, and he's right.