I just completed a free training video on the use of a free invite all browser add-on for facebook which mass invites users who already liked your posts, instead of you having to manually invite each one.
Invite All Chrome extension link:
Get Viral Autobots software and training:
A couple notes..
This tool is only for the chrome browser, there is no versions for other browsers available.
Whenever facebook makes updates, which can be frequent, it has broken the add-on, so it may or may not keep working.. the guys at Viral Autobots have been pretty good at fixing it when it does break due to a facebook update.