Many of his ideas were reintroduced in the 20th century starting with sociology, then with other critical theories. His ideas were influential to many post-modern thinkers.
Marx was a materialist
In Das Kapital, Marx describes the problems and pitfalls of capitalism that would lead to its failure.
Critical Theorists, especially those in the Frankfurt school, took Marxism and 're-applied' it to modern problems. They focused on Marx's equality and hegemony principles in neo-Marxism, and less on Marx's ideas about the failure of capitalism.
Marx and Mass Media: http://www.marxismmadesimple.esmartweb.com/massmedia.htm