Can you check if you're running any add-ons in either Firefox or Chrome that are meant to enhance privacy. Add-ons like "Ghostery" and others are specifically meant to prevent some connections to social networks and would likely be causing the issues you're having.
Alternatively, it could be a firewall on the system, but since it works in some browsers, my guess is that that's not the case.
When I tested, it was on a Mac using Chrome. We'll gladly test some more, but I think the issues seems to be more with your setup, and we'd need to nail down what's specific about it.
Thanks,
Alex