So now Procter and Gamble just announced that they are setting up their first ecommerce site, initially as an "experiment" rather than as competition to their partners. But I presume private-label brands started out as an "experiment" by retailers also.
Marketing research data shows that store brands have been making inroads against branded consumer packaged goods items. (E.g., see Progressive Grocer.) But statistics don't always tell the whole story. One thing reports like this don't show, though, is why consumers are switching. It would be interesting to find out how many consumers are deliberately choosing store brands vs. no longer having a choice.