Chaplin was not a communist.
From a generally fawning Wikipedia entry:
Chaplin again vocalised his political views in Monsieur Verdoux, criticising capitalism and arguing that the world encourages mass killing through wars and weapons of mass destruction.
Because of this, the film met with controversy when it was released in April 1947; Chaplin was booed at the premiere, and there were calls for a boycott. Monsieur Verdoux was the first Chaplin release that failed both critically and commercially in the United States.
The negative reaction to Monsieur Verdoux was largely the result of changes in Chaplin's public image. Along with damage of the Joan Barry scandal, he was publicly accused of being a communist. His political activity had heightened during Second World War, when he campaigned for the opening of a Second Front to help the Soviet Union and supported various Soviet–American friendship groups.
He socialised with known communists, such as Hanns Eisler and Bertolt Brecht, and he attended functions given by Soviet diplomats in Los Angeles. In the political climate of 1940s America, such activities meant Chaplin was considered, as Larcher writes, "dangerously progressive and amoral." The FBI wanted him out of the country,and early in 1947 they launched an official investigation.
Chaplin denied being a communist, instead calling himself a "peacemonger", but felt the government's effort to suppress the ideology was an unacceptable infringement of civil liberties. Unwilling to be quiet about the issue, he openly protested the trials of Communist Party members and the activities of the House Un-American Activities Committee.
So apparently he just walked and talked like a duck?