Please help... I need advice... All of my friends have abandoned me :(?

Please help, I've been feeling so depressed lately :( None of my friends want to hangout with me anymore. This time last year, I had a group of about 20 good friends from my school (4 of which were my best friends), but now it seems that they are all blowing me off. None of them invite me to hang out anymore. I see all over Facebook that they have big parties and get-togethers, but they never invite me. One of my friends even told me that her pool party was canceled, but had it anyway without telling me. I've been trying to make the initiative by inviting them to my house or beach outings, but half of them just ignore the invite and the other half show up but exclude me from their conversations. I've been careful to stay out of drama and I did nothing to hurt any of them. I don't mean to sound whiney, I just don't know what to do. Part of me feels really betrayed by them, but I also don't want to isolate myself from all of my friends either. Any advice?