99% of games on the market today have multiplayer. Bioshock, however, did not. It was geared towards a singleplayer experience. But the sequel? It'll have multiplayer.
At least, it will according to the latest issue of Game Informer. Problem is, the mag's big story on the BioShock sequel only says it will feature multiplayer. It doesn't say what kind of multiplayer. So while earlier rumours suggested co-op play would make an entrance in the series, in the absence of any real details you can speculate there'll be plasmid-fuelled deathmatches and nobody can say you're wrong.