Are you allowed to use Facebook at work?

Using Facebook is a prerequisite by the employer

I have to use Facebook, I'm the social media manager!

if you finish your work go facebook :D if you didn't go facebook lol im kidding go finish your tasks but its nice to have a5 min facebook break

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i think it's according to the nature of the job

Its totaly depend on job nature, when we are talking about social media its mandatory .

Facebook only for your private or personal network that is why no need in office or at work place,

Yup...
its allowed in my organization as i deal with sales function of my company.
Facebook & linked In i consider to be a good medium to interact with your clients, as you get an opportunity to understand him better by knowing their likes, dislikes etc....
these personal information helps you in making your pitch better...

It depends on the company your work in

If u need to research and gather insights , feedbacks and market surveys why not, digital mdia became essential part of our daily business , meanwhile, monitoring the concern employee' time on facebook might be quite difficult but if the employee role is not related to the social media portals then its easy to monitor.

We are allowed but hardly get manage due to busy schedule :)

if you are in media, then yes.
if you have requirement to measure social media then yes for such jobs like product manager or category manager.
the theory is that any web based browsing in the working place should be for the purpose of work rather than pleasure.
remember that each minute spent out of work context is a loss to the business and its deemed to be thieving from the company.
why? because they trusted the individual and paid him to work rather than to socialize and state what he feels at the time during working hrs.
if you read your contract of employment and the employee handbook, it all stated that no use of the internet for personal purpose and that answer the question.
many large companies do block the access by default to all employees and only unblock by request from the manager of the individual after explaining reason of use.