how to unblock facebook
What do you mean by unblock?
I am guessing by unblock you are referring to how to access Facebook and bypass a firewall which has been set by your network administrator.
If it is a home network then things are going to be easier then if it is an enterprise/business network. The simplest way would be to use a proxy which essentially connects you using an outside server and thus the filter sees the proxies website instead of Facebooks.
This will only work if the proxy you use also hasn't been blocked, which may be the issue with an enterprise or business network.
Now, down to the nitty gritty facts. If it is at work/school then attempting to bypass the security can land you in a heap of trouble including being fired... is it worth the risk?
If it is at home then your parents obviously have some knowledge in how their router works and so they also probably know how to monitor your web usage (no use deleting your browsing history... shock horror) and you risk getting your internet usage cut all together... is it worth it?