How many users use Google, Yahoo, MSN and Outlook Calendars?

I'm working on a product and needs to check how many users are using web based calendars. Or how many users create calendar events per year. I need this to figure out the market size. I tried to google those terms but did not turn up anything.

Worst case scenario, how can I calculate rough #'s if no concrete information is found?
Any help is much appreciated.

Off the top of my head, unless you can find a source that explicitly reveals this information, you can try and work backwards through their traffic data for the strictly web-based calendars.

• http://www.alexa.com/siteinfo/google.com
• http://www.alexa.com/siteinfo/yahoo.com
• http://www.alexa.com/siteinfo/msn.com

You'd need to find out the sub-domain associated with the calendar, then try and use that to filter out the traffic. Though it looks like Yahoo is the only one that still uses calendar.yahoo.com I believe, the others redirect so you'll have to do some research. A 'fuzzy' answer at best, but possibly better than nothing.

Alternatively, you could try digging through their SEC filings. Just a guess, but they may actually discuss user/traffic figures when talking about their products.

There's really no good way to figure this out based on web traffic stats (things like Alexa & Compete are highly inaccurate and even worse for something like this). Odds are that the only way to really get a solid estimation of this would be through a survey conducted by a market research firm (which can carry a substantial cost)