I agree with the first response. It seems you are 'sad' that your child is not included, there is no mention of how your child feels. It can be difficult to realize our children's circles of friends are changing and that not everyone circle will include everyone. As a mother of four, birthday parties are very expensive and if the child chooses to have the party at an establishment of some sort there are size limits on the number of kids that can be invited or the price jumps. I hate paying hundreds of dollars for a b-day party. I allow my kids to choose which friends they invite to their party, this can be very difficult for them as well. I explain to them that we are not able to invite everyone we know, you must choose only X number of all your friends. Some times we invite a certain group of kids, classmates, scout troop, church, neighbors. This is a great lesson in life, for both you and your children, we have several circles of friends and not all of them overlap, this doesn't mean that you are any less of a friend, you just don't fit into that circle.
Good Luck, I'm sure you will realize this as your children get older and they develop different groups of friends and you have to plan a party at Chuck E Cheese, Ice Skating Rink, Pump it Up, Princess Dress-up Party etc. It can be $200plus!