I asked a question during Matt Aslett's excellent presentation on Monday at the Microsft/SD Forum OSBC Zero Day event: Does your community manager report to engineering or marketing? Matt gently stepped out of the way, but there is exactly one right answer: Engineering.
This doesn't mean that the community isn't an enormous source of word-of-mouth evangelism for the company, the project brand, and self-qualified leads over time. But the community doesn't want messages and they don't want to be qualified or converted. The community is already setting their own expectations around the project instead of buying the product. Neither does this mean that marketing is out of the loop at developing inbound requirements from customers for the engineering team as they develop the software that feeds into the product.
While marketing traditionally managed the "developer network" in closed source companies, that's because the software wasn't a community engagement mechanism for users that weren't customers. Growing the developer community around your platform was a marketing function based on the business strategy of growing market share and providing complement value with lots of "knowledgeable developers" for customers. The software part of the solution wasn't a source of customer contribution, innovation, and testing resource. Your community equated to your customers.
There are companies that historically have strong product management departments that are often staffed by engineers that have crossed the floor to marketing. There's still a problem here with the community manager reporting to marketing, because the marketing department is traditionally measured on marketing functions. They will behave against how they're measured.
So community development and management is an engineering function.