0.0.0.0 is invalid, so should cause an immediate fail without attempting to connect. If you run a webserver on your computer, a loopback address may actually hit the webserver and require a response.
0.0.0.0 is linux format, 127.0.0.1 is another format and the one I use; the top of the HOSTS file determines which is used
There's some who claim "127.0.0.1 on some machines this may run minutely faster" http://someonewhocares.org/hosts/zero/
So 0.0.0.0 www.login.facebook.com would go to LocalHost or you -satisfying the request be it you or a link from a web page.
The HOSTS file is a routing file. you can also use for shortcuts like 184.108.40.206 findnow - typing findnow in the address bar would take you to Google.
A HOSTS file is simple tracking, malware protection that even speeds up page loadings by removing the non article stuff.