I introduced my neighbor (who was very close to me) to another friend and now they are doing things without me?

Sad to say but I think they have a deeper bond than they did with you. Consider yourself lucky to have found out they are the ones that are acting childish--by doing these get-togethers behind your back. This is something made for middle school. Apparently your pretty good at making friends so don't let this behavior stop you from missing out on life--get out and meet more mature minded individuals you'll make yourself a whole lot happier and less miserable. Good Luck Miss Socialite-I wish you well.