I think the point here is not on the amount of contributions, it is about: can you build a Twitter clone using exactly the same opensource technologies as Twitt

I think the point here is not on the amount of contributions, it is about: can you build a Twitter clone using exactly the same opensource technologies as Twitter, or how easy is it? Twitter mainly use open source technologies and they will contribute back if they created something useful - even the projects are the core components of Twitter. For Google, it would be much harder, as they use so many proprietary (backend) technologies, GFS, BigTable, Chubby etc.