At the top of the most popular open source CMS platform is Microsoft’s own Orchard Project (http://www.fligit.com/
All three of the open source platforms mentioned as of this writing has a Fligit score well above 300. Compare the open source scores with the commercial .NET CMS platforms, and you see that the online community for them all hover at a Fligit score of 200 or below, with Sitecore leading, followed by Sitefinity, and Microsoft SharePoint taking third.
The numbers appear to indicate that the commercial platforms do not have folks sharing information about their respective platforms, whereas open source systems have a large community following that are highly engaged in social sharing.
Patrick Santry, owner of Fligit, and four time Microsoft MVP Award recipient says "This shows that the Microsoft open source community is flourishing, and actively engaged not only with their respective platforms, but engaging their social networks about their favorite CMS."
For a complete breakdown of each CMS platform and how they are engaging each individual social network, visit the links provided in the article.