You don't need to "share" your PFB letter, it's only an option. As far as whether or not it works; it depends on the company, and probably even the person who reads your letter.
yes, it works, and you don't have to make your letters *public*. after you write your letter, on the last page, there is a box you check(or uncheck, i can't remember) if you don't want your letter to be made public.