Ferguson had called Webb "the best referee in the country" before the official took charge of United's 2-1 Premier League victory over Chelsea on May 8, while Ancelotti described him as a "fantastic referee" prior to the Old Trafford meeting.
FA regulations make it clear managers are not allowed to comment on referees before a game, regardless of whether the statements are positive or negative, and the FA said it wanted to send a message out to managers to watch what they say.
Regarding Ferguson's case, the chairman of the FA's regulatory commission said in a statement: "This is a clear breach of the rule and it is the first time such a matter has been placed in front of a commission."
"As a result of this charge, the commission were aware of other occasions where pre-match comments were made by other managers.
"In this case, it was considered to be a minor breach, but a breach nevertheless, and it should be taken as a warning to all managers in the future that any such breach, even positive comments, are likely to result in a charge by the FA."
Later on Tuesday, the FA announced it had also warned Ancelotti "as to his future conduct" for comments he made at a news conference on the same day as Ferguson's remarks.
Ferguson is unlikely to let the warning spoil his jubilant mood after leading United to a record 19th league title at the weekend but he may be more careful about what he says before Sunday's final league game of the season against Blackpool.
The United manager was charged with improper conduct over the comments, which came two months after he received a five-match touchline ban for criticising referees.
