Copyleft is a very demanding license in terms of what it dictates to downstream uses. The license’s goals is to infect users in a sharing ideology as well as to obtain the de facto result of having more code reused (or any other content). Whether this works is one interesting question, but it is also interesting to understand what the GPL entails for collaboration, rather than sharing.
Once actors are motivated toward sharing and reusing via the GPL, they are also motivated toward a social or participatory behavior.
The restrictive tone of the license undermines collaboration and participation.
No impact at all.
qualitatively: What does the GPL do for you. quantitatively: Comparing GPL networks of collaboration topermissive licenses networks of collaboration (ceteris paribus to the extent humanly possible, of course)