Hi everybody and thanks for quick answer.
It's true that the website was infected long time ago but it was at the beginning of 2010. Virus was removed successfully in a short period and everything worked normaly after that.
Suddenly on 20th October 2011 Avast started to block that website even though the files on the server remained the same. Since then I'm using MAC OS for browsing that website. I have also used other PCs with other antivirus programs and i could browse the website without any problems.
There is no any *facebook*.* file on the server for a long time. I have also downloaded all files from server, scanned them with Avast and I didn't found anything suspicious.
I have already reported site here
http://www.avast.com/contact-form.php?loadStyles few weeks ago but nothing happened since then.
Thanks once again for your help.