Is it only social networking? Or is it possible this is just what the majority of users are accessing and so that's the problem that is being reported? My hunch tells me all wi-fi traffic is affected and not just social networking.
In this case its likely a wi-fi issue like interference with other wi-fi networks, bad signal strength, over-utilized AP (too many clients, too much bandwidth) or other things.
If it's truly just an issue with Social Networking and other things work fine like streaming video, regular web pages, etc then look at the firewall for differences in policies between lan and wifi (i.e. do you have app control turned on for one but not the other)?
Also are you using the same network config on both lan and wifi? Do all clients use the same DNS servers, for example? If wifi is using different DNS try and different DNS server, etc.