If the "Login with Facebook" button is missing, and instead its just text, that's usually caused if the Facebook Javascript API can't be loaded. That easily could be from a proxy that is blocking access to the Facebook site.
Can you navigate Facebook.com normally from your office? If not, then I think your assessment is correct. Also, many firewalls/proxies will let 'anything' go through if the proxy is overloaded, so it becomes an intermittent failure (this prevents the proxy from slowing everything down, while still blocking most 'bad' content).
As for this being a normal thing, no, obviously not. Another thing you may want to try is coming to this site or our demo site periodically to see if you have the same problems. If not, then it could be a misconfiguration, but I don't know what would cause that off hand.
Keep testing, and let us know what you come up with. We're here to help.
Viewing behind tight proxy...
