When I try access a website, and it is down, I head to http://www.downforeveryoneorjustme.com, and test it. But lately, my home network PCs cannot access facebook.com, and I tried the that service and the answer was:

It's just you. http://facebook.com is up.

Ok, that got me. I tried several browsers and 3 PCs in my LAN and it don't works. I don't know how to troubleshoot this. What some step-by-step to troubleshoot that problem?

Output from ping command:

Disparando facebook.com [69.171.234.21] com 32 bytes de dados:
Resposta de 69.171.234.21: bytes=32 tempo=256ms TTL=245
Resposta de 69.171.234.21: bytes=32 tempo=255ms TTL=246
Resposta de 69.171.234.21: bytes=32 tempo=251ms TTL=245
Resposta de 69.171.234.21: bytes=32 tempo=255ms TTL=246

PS.: I thank you for the nice help, but then I suppose that the first step of a step-by-step to troubleshoot is ping from command line?