Police quickly descended around the finish line and, with cell phones mostly not working, Bozorg and her boyfriend repaired to his home, turned on the news, and logged onto Facebook, where Bozorg posted that she was OK and that her phone was down.

"I have been following my friend's Facebook [account] who is near the scene and she is updating everyone before it even gets to the news," Bozorg said by e-mail on Monday night.

Indeed, as word spread of the blasts on Monday afternoon, social media seemed shaped by every aspect of the response, from runners giving their accounts of the race-turned-nightmare on Facebook, to authorities using Twitter to give instant updates, to The Boston Globe temporarily converting its homepage to