Existentialism and Sociology

I found the section on Cornell West extremely interesting. It is clear his values and position were imperative to his theoretical writings. What is less clear is how well his existentialist ideas mix with sociology. Existentialist philosophy argues that there is no fundamental essence of humanity, but rather individuals’ essence is created through the lives they lead (Sartre, 1985). It seems though, most theorists, including Marx whom is imperative to critical theorists, claims that there is something fundamental to being human. This contradiction has bothered me personally as an existentialist and sociologist and seems that there must be a compromise between the two. It was somewhat comforting to know that there is another person who deals with ideas similar to mine, but I wonder:

Just how compatible is a school of philosophy that assumes there is nothing is fundamentally human and a study that often makes assumptions of human nature?