I worked for an exchange and it was not blocked there. I worked for a car automation company and it wasn't blocked there either. My following two jobs were at multinational banks. In the first one, it was blocked. In the second, I took a "social media responsibility" training and I was added to the exclusion list and I could access Facebook, Reddit, Twitter etc. Now I work for a small finance company and the network is wide open.

No. I work at Facebook, and we don’t block any sites except ones with malware.

No it is not but I remember a hospital in Central London which Facebook was blocked in. I'm not sure if it was a policy or they had an issue with their network!

No, but Google Plus is, which is where Google Hangouts stores videos. Immensely frustrating during my lunch break

Today I'd prefer work in a place where most social media sites were blocked (except Hacker News?).

Nope, over the last 5 years i've found employers more leniant

Yes. Both Facebook and Twitter are blocked.