hotmail is the most popular web based email in the world, internet explorer is the most popular browser in the world, bing wich also powers yahoo accounts for about a quarter of searches in the US, and ten there is windows live, azure, skydrive, etc. Much bigger than most significant web operations imo
How are they significant: by the profit they bring, by the market share or by the investment made by the company?
Personally I never associate Microsoft with the web. It's either desktop OS or Xbox.