To use social media in your work is beneficial or it is dangerous?

In my view it is not advisable to use social media at your work. Social media is not private, anything you share about your work or company can be visible to customers, shareholders, or other employees of the company. Any small information, rumor you share in social media can create bigger problems for company.

People justify using linkedin which is more specific to professional network rather personal information, or statuses. But, people need to refrain themselves from sharing anything about their company or work in any type of social networking.

with the idea of someone wanting to bully me at my workplace (either known or unknown) ..... off course no to social media

It depends on what kind of work environment you are working, there are many corporate social network like yammer, Tibbr, socialcast, jive, and many other, which can be beneficial to get connected with your other colleague, open discussion on any issue, get mins by mins update on any issue or story, find solutions already discussed on any specific issue. 

Depending on the type of work you do. If you're involved in social media advertising, media sales and other online products, Social media would be a part of your job description. However, if your work is not related to the media field, social media would be reducing the employees efficiency and costing the employer valuable time and money.