The council calculates staff spend between five and 10 minutes a month on Facebook - 400 hours a month across the organisation - and has decided to curtail the usage because it was unable to ascertain if staff were using the site on breaks or during work time.
The ban will come into effect over the next few weeks and YouTube has already been blocked.
David Williams, chief executive at Portsmouth City Council, said: "400 hours spent on Facebook per month sounds like a lot. However, when looked at in the context of approximately 4,500 users this could equate to a monthly average of less than six minutes per user.
Staff are aware they should only use their computers for personal use in their own time - such as on a lunch break, and managers enforce this policy. We regularly revise our position on this as the internet environment is constantly changing.
"We revised the policy in July to facilitate work-life balance and allow things like internet banking in staff's own time but we intend to restrict internet access to social networking sites more than at present for non-business use. Any member of Portsmouth City Council staff may, under this revised policy, make a business case to have these sites unblocked if they need to use them for Portsmouth City Council business."