Market Share and Webmail

I had a thought this weekend listening to some podcasts on technology news. There’s pretty consistent bitching about Microsoft and Yahoo and Google and market share. I wonder how realistic of a consideration market share is for companies like this. I know that for myself I have accounts with all three, MS, Y! and Google. In fact I have multiple accounts at each as well as free web-based email accounts with other sites that I use for different things. In the case of many younger people, having multiple email accounts is fairly typical, having a hotmail account and a gmail account is reasonably common. In these cases, how much does market share really matter? And what represents the share of the market? Is it determined by how often I use one service versus the other? Is it based on which email I check first? For companies like MS, Yahoo and Google, I don’t think you can properly gauge the effect of a company by what some analyst or poll says about it’s share of the market, because users regularly find themselves as customers of multiple vendors.