Add this to your NixOS Linux configuration.nix:
{
networking.extraHosts =
''
127.0.0.1 reddit.com
127.0.0.1 www.reddit.com
127.0.0.1 old.reddit.com
127.0.0.1 www.old.reddit.com
127.0.0.1 news.ycombinator.com
127.0.0.1 hckrnews.com
127.0.0.1 lobste.rs
127.0.0.1 www.lobste.rs
127.0.0.1 twitter.com
127.0.0.1 www.twitter.com
127.0.0.1 facebook.com
127.0.0.1 www.facebook.com
'';
}
(On Windows, the file to modify is c:\windows\system32\drivers\etc\hosts.)