How to say this?
You signed up for Facebook but you don't want to use it anymore, so you're going to do this to make your ID no longer valid.
In this case, what do you say? Close my Facebook? Or withdraw? Cancel?

Question

Answer

Ne careful that you don't permanently "delete" your account.

Deactivate = temporarily disable. 
Delete = permanently terminate

Answer

Do you mean you "sign out/log out"?

If you mean you are not going to use your FaceBook account anymore it's "deactivate"

"I am going to deactivate my FaceBook (account)."

Não perca a oportunidade de aprender uma língua no conforto da sua casa. Navegue pela nossa seleção de professores de línguas experientes e inscreva-se na sua primeira aula agora mesmo!