Sounds a bit like a natural progression: they didn't know anyone, you and your husband very kindly introduced them to others, and now they are also friends with those people. So you should feel that your efforts were a success! Their son hit it off with your son but also with 2 other boys whose parents, perhaps, were a better "fit" with this couple. You had other friends when you met this couple (because you say you introduced this couple to your friends) but now it seems you are a bit hurt by the fact that they have their own friends.
I do think you are reading into it too much. Maybe they just have better pictures of these other kids? Maybe their son doesn't see them as much as he sees your son? Maybe you are counting the playdates a little too much, and maybe you aren't seeing the times that you spend with other people?
You can do one of 3 things: You can comment on the friendship wall (assuming it's in a totally public place rather than in a back hall where you shouldn't have been!) and say to the mom, "What a nice thing to do! Oh look, there's Brandon. Oh, and here's Jimmy!" You might add to your child, "Let's see who else is up here! How many kids can you name?" like it's a memory game. Another choices is, you can provide a photo of your son with their son, and donate it to the wall. Finally, you can ignore it.
I looked at your last few questions, though, and I see a pattern of being very involved and extremely sensitive about young children's friendships and who are the BFFs and who are on the periphery. I think this may be more of a problem for you than for others, and perhaps more than your realize. I'm concerned it's not going to be healthy for your kids if you don't put the brakes on some of these persistent and somewhat obsessive thoughts.
I think the surest way to break up a decent friendship is to get so upset that the people (adults or kids) have other friends. You're much better off developing and cherishing a diversity of relationships.