Google Calendar has surpassed MSN Calendar and is rapidly approaching Yahoo Calendar in U.S. visitor market share, according to data released on Wednesday by Internet metrics firm Hitwise.

Hitwise said that the number of visitors to Google Calendar had increased threefold since June.

Unlike MSN Calendar or Yahoo Calendar, Google Calendar has been getting significant traffic from non-Google properties.

"Yahoo Mail was Google Calendar's third largest source of traffic, with 2.7% of upstream visits, and Hotmail was sixth at 1.2% of visits," explained LeeAnn Prescott, research director for Hitwise, in a blog post. "What this indicates to me is that people who are not necessarily heavy Gmail users are now using Google Calendar, including users of Yahoo Mail and Hotmail. Its easy-to-use features and sharing capabilities have allowed it to steal share from its competition, and in this case, catch them in less than a year."

Google, which has been working assiduously to broaden its appeal beyond search, surely welcomes this news. But for Yahoo, which has led Google in online services outside of search, this has to be a troubling trend. While Yahoo Mail, Yahoo News, Yahoo Finance, and Yahoo Maps all enjoy significantly more market share than their Google counterparts, the success of Google Calendar makes it clear that users aren't as loyal as online companies might like.