is there some advantage to having them. I think I have about 5 of them right now but not sure what that does for me.

Having Friends comes in handy if you do different things around the site.

As a Gold Key member, when I post a book, I can see if any of my Friends are wishing for it, before I post it regularly. (So, I can check a stack of books to see if there are any Friends to whom I can send multiple books, thus saving postage).

It also comes in handy if you play in games, because you can send PMs to multiple friends together.

Also, I use Friends to keep track on members who either had interesting bookshelves (that I want to check out later) or who had Wish Lists that I could post multiple books to.

So, my Friends generally consist of people with similar reading interests, or people that I play in games with (who are also likely to be people with similar reading interests).

Is there any way to go about sorting users based on similarities? I think I need to rate more books, but I always seem to have a pretty low compatibility score.

I just use friends to see if they have any books on their WL and I haven't posted them up yet. Just recently, I have added mine depending on swappers who notify me quickly in trades and say thank you.

Catheline, you might seek out like-minded readers in the various genre forums, to find folks who like things that you do too.

I tag my friends. So there are different tag clusters. This lets you PM just the tag group. So I can send a PM to folks from swap games. Let's me find the members that I have marked as giving me a great deal on children's readers. 

Like others, I friend people who have interesting bookshelves or similar reading tastes. Some of them are friends...we've exchanged PMs about an author or genre and have gotten to know each other a bit and will share new authors that we have enjoyed. I too am a gold key member and when I post a WL book, I would much rather post it to a PBS Friend because they will usually accept it promptly as opposed to having the request roll thru the general population, where I have often experienced a WL book going thru multiple members before it is accepted. Because I am posting WL books to friends, I also am offered WL books faster as those same friends will post WL books directly to me too. Having PBS Friends has definitely enriched my experience here!

Those tags are awesome and Pming the tag group is even more awesome.  I don't have much cause to use it here but on a sister site I use it when I host a game.  Really makes things easier.  

Friends are what you make of them in life or in pbs. Some of mine here have become close friends for various reasons. We begin with a swap and pm to find out more about interests, etc. Last year I actually met up with two pbs friends that I had a special connection with: First, we all live  in the same state, although in different areas. Second, we are all close in age, even our birthdays are close together. We met at a beautiful museum in a centrally located city and spent the day together. There was a great restaurant in the museum where we took a long break for a good meal and non-stop talking, and even exchanged books with each other before we departed. A fun, memorable experience.