The current thresholds for FD are as follows:
Silver Elite 25,000miles or 20 segments
Gold Elite 50,000miles or 40 segments
Platinum Elite 75,000miles or 60 segments
I don't know what exactly are the current differences in benefits between Gold and Platinum but I remember reading that Plats are allowed two guests in the lounges while Golds are allowed only one.
About the bonuses, I am not too sure about the differences.