How do you manage your employee behavior when they use social media during working hours?

I think if someone is prompt in his/her job and on daily basis they are doing their work complete so its dose not matter they are using social media on duty time.  

There must be a clear cut instruction about using the social media and it should be monitered and implemented accordingly.

Employees spend at least8 hours in the office. It would be lame to expect them to work and be productive for this long duration of time. Taking short breaks every couple of hours to interact with others, either in the office or on social media, will increase their productivity, improve the quality of work, and give them more satisfaction. So as long as their tasks are done well and in time, no need to suffocate them.

The best solution is through restrictions imposed by the IT department. The firewall will refuse giving access to those websites.

You have also to monitor the firewall logs from time to time to see if any employee is using the net for fun for long time.

Through predefined rules which determine the penalty related to any unethical issue

By making work task-oriented rather than8-hours oriented, ie,finish your work and walk free 

Facebook. LinkedIn. Twitter. These and other social media sites have created fast-paced opportunities for individuals to network and for information to spread. However, with these opportunities come potential hazards, particularly in connection with the workplace. By formulating social media practices and policies that comply with recently issued legal standards, employers have the ability to minimize, if not eliminate, the challenges posed by employee use of social media.

Job responsibilities shoulod be clearly defined for each employee and they should be assigned specific tasks to be completed and reported complete within a specified time period.

In my opinion, as long a the employees are performing their job responsibilites well, it hardly matters if they spend some time on social networking sites even while in office - but with limitations. . 

http://www.mediabistro.com/alltwitter/files/2013/11/social-media-workplace-study.jpg

Not longer then10 minutes after a short break / during a short break. A recent germany study showed, that people who used social media feel more connected to their friends and are more happy. Hence their productivity were higher. 

The good point to take the advantage of using the social media to boost up the business through themselves.

It is a play of emotional intelligence to direct the team as per the company policy with less anger against them.

The happily secured life reflects positively to the team, I think that you know the google experience to add value to their employees by joys and intertainments inside the work station.On the other hand work should be done on time and keep them punctuated

There are couple of web filtering applications which work with firewall to generate reports about employee internet usage and also block traffic per site or application. So you have option either you silently cut them off or issue warning.