We can argue that multiculturalism has decreased social cohesion because it has encouraged minority groups to refrain from assimilating into the mainstream society.
In past times, it was assumed that immigrants would assimilate. They would try as hard as they could to become just like other Americans. This led to a situation where there was strong social cohesion because almost all people held the same values and shared the same culture. With multiculturalism (one can argue) immigrants and minorities are encouraged to remain separate. They are encouraged to hold to their original cultures rather than assimilating. This creates a society that is split between people of different cultures, rather than a society that is homogeneous and cohesive.
I would say that multiculturalism as added to the fact the people now joined together and try to be united. More so than ever people communicate and join together as one giant community. Of course there were still prejudice and people to choose to isolate themselves but I'd say on a whole people nowadays try to unite.