Check this page for the official Facebook usage statistics. I found it especially interesting that:
Do you still test your website performance first in IE and only after this in Firefox? It seems like it’s time for changing this order 😉 By the way, Chrome is n3 (with 17%) and it’s only the beginning.
Here goes the link to the complete report http://www.w3schools.com/browsers/browsers_stats.asp.
However don’t forget to check your website statistics to know what browser is most preferred by your website users.