Infamous: Second Son will not feature a multiplayer mode, according to the game's brand development director.
Ken Schramm clarified the situation in conversation with Taiwanese site UDN (via Dualshockers), where he explained that while Delsin will meet multiple NPCs throughout the course of the game, it's designed as a single-player experience.
Elsewhere in the interview, it was revealed the game has an open architecture, so you can do quests outside of the main story at your leisure. The one caveat with this is that certain situations only appear if you're aligned a certain way, so if you make certain choices you'll lose access to other situations forever.
This is good news to me, I have always been a Single Player gamer. I'm glad that the game is focusing on the single player aspect like story and missions instead of another tact on mediocre multiplayer. Many developers just include a half baked multiplayer component just because it's popular and sacrificed efforts on single player.