Edit: Replying to OP, not donoreo
said by donoreo:
The issue is a personnel one, not a technology one. If people are spending too much time on these their manager should be taking care of the problem, not technology.
Again - for emphasis...
On a related note - depending on your policy with personal devices (not the mobile company ones), I've created a separate wi-fi only network users can use for their personal items. Throttled and heavily restricted (time limits, port 80 only), it doesn't touch our internal network - so if they want to go ahead and waste away on it - more power to them.
When things don't get done, you can provide logs on individual users showing management how much time they spend on particular stuff.
If management wants to go full blown open up the gates - get it in writing if you're against it and your advice was not heeded.
Oh, and don't forget to include the overhead your dept. will face with dealing with setting up security etc... to monitor this.