When you are contemplating starting a new software company you want to look at where money gets spent now and where it's going to be spent in the future. That's why startups these days are building their applications on utilities like Amazon S3 (which despite last week's outage, I still believe in) and attempting to monetize Facebook (I am not a big believer in this one though I get the idea.) And while neither of these things may be right, they are better bets than building your infrastructure on dying platforms or betting on outdated technologies.
One product that I use daily is SpanningSync, a simple sync utility that connects iCal and Google calendar. One of the guys there (Charlie is his name, but I couldn't find his title or role) has a very logical and eye-opening take on why they went the way they did.
I didn't know how many users Google Calendar had, but I knew it was a large and growing number. But at some point it occurred to me that Salesforce had fewer than a million users (and perversely, only a fraction of them are even allowed access to the Salesforce API), and while their growth was impressive, that absolute number of users was still relatively low.
The other platform I used on a daily basis--Mac OS X--had roughly 25 million users with growth that CNET called "nothing short of spectacular". So I put the Salesforce sync project on hold (and later passed it off to a consulting firm) and shifted my focus to syncing Apple iCal and Google Calendar. That turned out to be a good decision.
This is a good point--go to where the users are going. A larger potential audience means larger potential dollars.