A best friend is there for you no matter what you did or where you are.  You probably would do the same for them - though the relationship does not demand this level of reciprocity.  Best friends are there to help you because of your existing/historical relationship.  Best friend's try to help you in your time of need, even when you don't want or even recognize the need for their help.

I don't know that brands can ever be best friends. 

A good friend is there for you when you need them.  They care about you.  They would come out to Vegas to get you out of trouble, and be there to share in your moments of celebration.  They in turn, would probably expect you to do the same.

Most brands are not capable of playing that role (excluding insurance companies).

A casual friend is there to help you out when you need to paint a room or move your furniture.

Most brands can play that role.

An acquaintance may be someone you recognize but have little relationship with.

Many brand currently play that role.

A familiar face is someone you recognize at a party, but can't quite place.

This is where many brands currently play in social "media".  You may recall bumping into their facebook application or group via a friend's participation, but you can't quite recall what it was all about.

A stranger is completely unfamiliar. 

Few brands can afford to live in this space.

Relationship marketing/social branding isn't just a question of if or when, it's a matter of how and why.

photo credit here