this is a simple facebook page(just as an example) and I want to know what is the name of the font which is used for the heading("Introducing Graph Search");
I can see the source of the page in google chrome by using the "view source" option in chrome but I can't just find the exact font name;
Use the inspector Computed Panel (Chrome)
font-family: 'Freight Sans Bold', 'lucida grande', tahoma, verdana, arial, sans-serif;
If you wish to use firefox plugin you may go for "font finder" plugin which gives good results on web pages.
https://addons.mozilla.org/en-US/firefox/addon/font-finder/
it's set in css file. And you should search style not only for class applied on this element you also sholud remember that css can use inheritance.
This font uses facebook