The other key thing to remember is that both GitHub and BitBucket are vulnerable to displacement if they "turn evil", and they know it.
While the two big free-as-in-beer proprietary hosts (and their network effects) do suck a lot of potential users and contributors out of the pool for the open source hosting options, GitLab, Gerrit, Allura and Gitorious are still excellent options for projects and organisations that are concerned about the governance of their dependencies.
With services like Red Hat's own OpenShift offering to handle the hosting and server maintenance component for free, a couple of those projects teaming up to work on a "federated pull request" system could potentially make inroads into GitHub's dominance.
The current state of the DVCS hosting market is like an email system where all of the email providers offered POP3 and IMAP to their users, but none of them implemented SMTP to communicate with users of other email providers. The two proprietary providers have no incentive to collaborate on a federation protocol for pull requests, so if it's going to happen it will need to come from the smaller open source providers.