Welcome Shaiamtokh,
If you've really just posted something critical or negative about Islam on FB(Facebook) while your still in Pakistan, then you don't need me to tell you that you've done something incredibly risky.
If the people on FB know you personally and know where you live then you might well genuinely be in danger.
As Yeez says remove it, and renounce it.
Try apologising, saying that you realise you made a mistake, that the things you posted tricked you and they are all false.
Maybe with a bit of luck people will leave you alone after that.
If this fails you might want to consider getting out of Pakistan fast. Some of the Mods on this site will probably be better able to advise you on how to go about doing that than I can. In the mean time have a look at this link.
http://ex-muslim.org.uk/2010/09/apostasy-and-asylum-in-the-united-kingdom/Stay safe.