I think a big difference though is geographic segmentation. Pepsi could dominate the Southwest and Coke could dominate the Northeast; there's no reason why that would be true of amazon vs. buy.com.
We do see that with internet companies only for national or language boundaries, e.g. baidu in China, gumtree dominates in the UK vs. craigslist in North Am, Yahoo beats google in some asian markets, Orkut is popular in Brazil, etc, etc.
We do see that with internet companies only for national or language boundaries, e.g. baidu in China, gumtree dominates in the UK vs. craigslist in North Am, Yahoo beats google in some asian markets, Orkut is popular in Brazil, etc, etc.