Tricking Facebook with custom thumbnail

hi guys, I want to get custom thumbnail for my link posts on facebook and i've two way to do that

- Facebook Multi Page/Group Post script

- Open Graph Protocol (html page with redirect)

btw there are two things i don't understand:

- why if i use FMPGP custom thumbnail my traffic seems to arrive directly from facebook (when i use Open Graph instead traffic arrives from the site where i uploaded an html file with og tags)

- why using OPG everything works fine except caption (site name) which still remains the original