I think sourcing product insights has its advantage on social media (open source) as well as developing a specific community. Asking questions and for product insight through any social medium channel that your followers are using will likely yield you some results, hopefully a lot of results. The larger the community, the more opportunity you have to capture answers, thoughts and suggestions that could help your company develop a better product or offer a better service.
Depending on your community, however, creating a separate opportunity for your special followers to participate in product insight may be a great option. Superfly.com did just that. They created a special community for particular frequent-flier program participants, which gave the members a sense of being in an elite club. The frequent-flier program members know the pain of long and frequent travel and were able to offer the company important insight.
I think the choice depends on what the company goals for the product insight are. Is the company looking for a wide range of ideas and suggestions or does the company have a specific focus for a specific category of follower? Once that is determined, then the choice of crowd sourcing the company’s social media channels or creating a separate community can be made.