A best friend you can always relay on, if your actual best friend is not around, or not available for the time being. You often talk to them on facebook or text them a lot.
Second bestness is when you know you are not the first choice if somebody asks you to be someone (wife, husband, girlfriend, boyfriend) or to do something (i.e. become the boss). You know that someone else was first choice and altough you accept the offer, it rankles and stays wth you forever, turning you into a bitter, overbearing person.
"I think Mum's feeling of second bestness is why she's always so horrible to me - knows Dad wanted to or should have married someone else."