I just had a quick look at a email list I run for a local sporting organization and it looks like the big 3 are less than 50% of addresses. There were a lot of local ISP addresses, businesses and educational organizations.
2 things to consider. A lot of people use more disposable accounts for mailing lists. And a lot of businesses and educational orgs use Google apps or MS's equivalent with their own domain name.
Are you basing the 50% based on what is after the "@", i.e. gmail.com and yahoo.com? Or are you looking at what server the MX points at. It's very easy to have your own MX for your domain point to a google mail server.
