Yes, the notion of proxy servers has been around almost as long as the Web itself, and students can easily find the location of dozens of these services that are used to circumvent Facebook (and other objectionable content). It takes about a minute to type in a Google search and load up your URL in their handy forms and off you go, block or no block. Certainly, some network admins are more diligent about blocking these proxy sites, but given the number of them, it is a losing battle.
I know something about this first-hand, having taught a high school networking class back in 2001-2. Back then, we had hard-wired PCs in our networked classroom, and few of the kids had their own laptops. It was a very simple matter to walk around the lab and pull the Ethernet plug out of anyone's computer who was surfing somewhere they shouldn't, and after a while, the mere threat of pulling the plug was enough to increase peer pressure to stick to the day's instruction. But now we have universal Wi-Fi and more kids toting laptops, so what can you do?
The best advice isn't to block, but to slow things down. Many network admins that I have spoken to over the years use some kind of WAN optimization/firewall appliance to detect these destinations such as Facebook and AIM and allow access, just very slow access. You can dial down the speed to specific sites and protocols, and make it something that will take just long enough that most kids will tire of waiting for the page to reload, and move on to their legit studies.
We covered these WAN optimization and control technologies in our story here.