[learc83]

Just because they have competitors doesn't make those competitors viable. Apple and Linux still existed when Microsoft was prosecuted by the DOJ.

The vast majority of people searching classified ads are searching craigslist, therefore if you're trying to list something in a classified ad, you're forced to use craigslist.

Sure you could use another service, but Netscape could have also just sold browsers only to Linux customers. It's all about the numbers.

[genwin]

But Microsoft largely escaped the DOJ antitrust case because of competition with Apple. (MS invested in Apple to retain a competitor.)

People aren't really forced to use craigslist. That the vast majority of people choose to search CL isn't good enough. Another company could spend whatever it takes to get people to search their classifieds instead. As long as CL can't or doesn't block that (in contrast to stuff MS was doing that started the DOJ case against them), the competition is viable.