I was having a problem with another identical type of extension that does the same and was having some big problems with it. I looked at parainvite and pro, saw the reviews and i have to say, they did put me off a bit.
Before even buying i sent in a question to ask that if i could not get it working would they go into my site and sort it out, the response was yes. I thought I would give it a go.
I am happy to say that everything installed perfectly and worked straight away! AWESOME!
I think the app could do with a couple of additions like being able to type someones name into the fb invite instead of having to scroll to find them but it is working very well for me.
So long as you follow the instructions you can't go wrong.