The Windows hosts file offers a great way of blocking or redirecting certain Internet hosts. I'm for instance using it whenever I move websites to a new hosting company to check the life site before the DNS has fully propagated. You can also download software like Hosts Man that allow you to add lists of known malicious sites or advertising servers to the file to block those automatically from being visited on the computer.
In theory, you can add any domain, host or website to the hosts file so that it is blocked on the system. Ghacks reader SGR just informed me that this apparently has changed in the Windows 8 RTM version.
While you can still add any host you want to the hosts file and map it to an IP, you will notice that some of the mappings will get reset once you open an Internet browser. If you only save, close and re-open the hosts file you will still see the new mappings in the the file, but once you open a web browser, some of them are removed automatically from the hosts file.
Two of the sites that you can't block using the hosts file are facebook.com and ad.doubleclick.net, the former the most popular social networking site, the second a popular ad serving domain.
The strange thing is that even write protecting the file does not have an effect on it as entries are still removed once you open a web browser. Actually, any kind of Internet connection seems to be enough for that behavior. If you open the Windows Store for instance, the entries get removed as well automatically.
This could be a bug that is affecting only some high profile sites and services, or something that has been added to Windows 8 deliberately. We have contacted Microsoft and are currently waiting for a response from a company representative. Since it is Sunday, it is not likely that this is going to happen today.
It is also in the realm of possibility that the hosts file may not accept other hosts.
Update: Tom just pointed out that turning off Windows Defender, which basically is Microsoft Security Essentials, in Windows 8 will resolve the issue. It appears that the program has been designed to protect some hosts from being added to the Windows hosts file. To turn off Windows Defender press the Windows key, type Windows Defender and hit enter. This launches the program. Switch to Settings here and select Administrator on the left. Locate Turn on Windows Defender and uncheck the preference and click save changes afterwards.
Please note that this turns off Windows Defender, and that it is recommended to have another antivirus program installed on the system to have it protected against Internet and local threats.
If you do not want to disable Windows Defender completely, you can alternatively add the hosts file to the list of excluded files and processes. You do that with a on Settings > Excluded files and locations. This basically blocks Windows Defender from scanning or manipulating the hosts file in the operating system.