Open sourcing their projects is good from a PR standpoint in this climate, and if anything it's a good way to share knowledge that would otherwise be kept within the company walls with those who need it (e.g. implementors of PHP/other VMs).

Creating and maintaining a community around an open source project is a lot of work, and it's understandable if Facebook doesn't want to pour many resources into this particular project, even if they keep it open source.