I think that multiculturalism might actually increase social cohesion.  Prior to the emergence of multicultural notions of identity, social cohesion was conceived in a manner where culture was not defined as relevant.  Culture was not stressed in the manner where it could be seen as part of a person's identity.  While there was a "oneness" being preached, the reality was that exclusion was still evident and there still was a sense that culture was relevant, even though it was being stressed that it wasn't.  For many, this resulted in relegation of voice and in experience, detracting from social cohesion.  With multiculturalism, the reality is that those who could share common cultural experiences could develop a sense of social cohesion.  The emergence of cultural identity as part of one's experiences or narrative that could be articulated and openly discussed helped to bring individuals together, creating social cohesion.  Perhaps, the multicultural element brings about smaller and different elements of social cohesion and not necessarily one grand vision.  It is in these smaller expressions of social cohesion where multiculturalism finds its most profound and lasting effect.