One big difference is consumer vs enterprise / B2B products. Or to say it better, social products that need a network (eg Facebook, Twitter) and for everyone to use the one market leading service vs selecting a specific best product for your personal or team needs.

Most of us probably agree that having different task managers, bug trackers, CRM systems, programming languages, browsers, ERP systems (or even watches, monitors, displays, cars, t-shirts etc) is good. And there small improvements in me-too products can be great.