Twitter said the news of Sir Alex's retirement received more than 1.4 million mentions in the first hour after the story broke at 9.18am.
It compares with one million mentions in the four hours after the death of Lady Thatcher was first announced last month.
The original tweet from Manchester United was retweeted or made a favourite tweet more than 18,000 times, while the official club hashtag #ThankYouSirAlex had more than 100,000 mentions within the hour.
Sir Alex Ferguson's brisk way with words has influenced the way we talk about football.
A timeline of the life of outgoing Manchester United manager Sir Alex Ferguson.
A special photo gallery of Sir Alex Ferguson's career at Manchester United as he steps down from his post as manager of the club.