Great Firewall (Internet censorship system of Chinese gov) use DNS
hijacking, IP blocking and keyword IDS (Intrusion Detect System) to
prevent you accessing facebook, twitter, youtube and more.
DNS hijacking : the DNS nameserver controlled by Chinese gov will give
you an incorrect IP of one website, and with DNS cache it will poison
all the internet.
2. IP blocking : some server IP (twitter for example) are totally blocked, there is no way but to use proxy to access to it.

3. keyword IDS : This is the most advanced part of GFW which is
developed by Cisco, In the blacklist there are many sensitive keywords,
if one some sensitive keywords are detected, the TCP/IP connection will
be reset by GFW, and your brower will give you "The connection was
reset" message.
You can use free proxy such as tor, freegate, wu jie to unblock facebook.
Also, you can use VPN to establish encrypted connection to unblock these sites, and prevent the censors from watching you.
You don't have to install any software on your system by using PPTP VPN
which is supported by Windows/Mac/iphone/android, and you can get it

Some free VPNs:
1. Air VPN
2. Angry VPN
3. AnonymousCoat
4. Arethusa
5. Bestukvpn
6. Bouncee
7. DearVPN
8. Free Canada VPN
9. Freesslvpn