Making friends is turning out to be important in MRA2, leading to unlocking at least two rares (Pickle[20] and Rainbow[50]). Here are some tips and guidelines:

-The count of your friends is on the main breeder screen.
-Each of your monsters can have up to 10 friends.
-You can make the same friends over again with a different monster.
-It is easiest to make friends (without a link cable and a real friend) on expedition.
-How well you beat a wild monster makes friendship easier. Beat it soundly and you'll more than likely make friends.
-There are different monsters for each rank so sometimes it pays to go on expedition earlier in life, besides the fights are easier.
-A retired monster can make friends. Even though it has only 50 energy, you can go to the nearest building in Promiass and search a few times (see next note).
-You don't have to successfully complete an expedition to keep your friends. Getting lost, or running out of energy does not matter.
-To make friends by linking, monsters must be within two ranks of each other.

Good luck in making friends, currently I have 55, let's see if anyone passes me. I plan to test to see if more rares will trigger at 60, 70, etc.

Nothing for 60 friends

Nothing for 100 friends either. Lisa teased me into going for 200, but I seriously doubt there is more forthcoming from the Soulmate Advancement Society.

I have a lot of new facts about making friends:

Item #1
The Link cable is the BEST way to make friends.
Thorr and I ran through whatever we had frozen and we each ended up with close to 30 new friends. How can this be? well we know the same monster can count more than once if he is friends with another monster
so, if Thorr had 4 monsters and I had 4 monsters we could each get 4x4=16 new friends if all went well. Also the link cable friendship test seems more forgiving than the one on expedition. Certain breeds didn't want to get along, but we think that was more a question of loyalty.

Item #2
Take advantage of the fact that the list of Wild monsters is different for each rank. It also seems to vary between ruins too (with some overlap).
One of the best times is if you are rasing a monster who does not need to live a full lifespan (like the Tigers or arrowheads you raise to make a thoroughbred). If you simply take one trip per rank starting at D and even if you only make2-3 new friends per trip. you end up with 8-10 new friends by Rank A. Also going once per rank and getting new friends is easier than waiting and trying to get all the rank A wild monsters. You end up getting repeats after the first couple so its better to start early then move on.

Item #3
I am going to eat crow and reverse my position on the Popular characteristic. It really did seem to make getting friends easier and more common. (see my post in the exploration thread for another benefit)

Item #4
There are 3 known levels of relationship, Thorr and I got one we had not seen before yesterday.
Friend- when you try and fail the monsters are friends. No move learned
Best Friend- when you try and get the light bulb. Monsters learn a friend tech and may receive some other personality change.
Soul Mate- we only managed this one between two trained monsters over the Link. No visible personality changes and the friend techs aren't any better. No idea what the difference is, unless there is some intangible benefit. Since we both had healthy hapy monsters, its hard to say, there was nothing bad to have the realtionship improve upon.

I plan to still work on my friend total, but not as diligently. Someday maybe I'll make up some nasty monsters to see if we can get in teresting results over the link.

You said that you can make the same friends with different monsters, but you can use the same monster too. My Angel {Virtue} made best friends with Bryon twice and both counted toward my total 9. It gave the same defensive move too.

That doesn't always happen.. Maybe if you met Byron in 2 different locations, but I have run into the same monster 3 times on one trip and it only added 1 to my friend count.

You also need to be sure you are reading the count amd not just counting your friend list

The count is on your first breeder info screen, just under your cash info