I think your parents are right about the summer holiday and these people will get back to you later on.
I can definitely understand where you're coming from, though. It does hurt and makes you wonder, I tend to take it very personally and it shakes my confidence quite a bit, it's difficult not to see it as a personal rejection. Unfortunately I haven't quite learned to let go of that just yet myself, so I struggle with it occasionally and I usually avoid initiating contact.
But you also have to accept that people lead their own lives and if they don't respond to you quickly it might not have anything to do with you personally. Sometimes people just need more time or they simply forget without having anything against you.
This might not be that easy to do at first but try to let go of the expectations when you do send people messages. You've done all that you could have done, you've contacted them and showed them that you care for them, which was probably your intention in the first place. This is your way or showing that people matter to you. If it comes back to you, then that's great, but if it doesn't come back in the same form, that's alright as well. Taking it personally is only going to hurt you, so letting go of that is just something you have to learn to do.
I always delete people from such websites after a while if I'm not regularly in touch with them. I don't see a point in having 500 people in my friends list when I hardly ever speak to any of them. I used to have that many in my friends list, but after a while I deleted more and more people. In the end, I also deleted all ties to my "original" family members (I had 67 friends when i was done).. I don't like having annoying people on there who cannot appreciate what I say or do, in any form, because if they cannot appreciate that then they are not really my friends in the first place. I say or do whatever I want, I joke and fuck around as much as I want, and nobody is going to tell me not to when I'm perfectly within my right to do so (Freedom of Speech right?). Perhaps you should consider looking at it the same way I do, or perhaps that would be too pessimistic. I find it highly optimistcal to burn bridges with these people though. I don't really like people all that much unless I hold them very dear. If you were an old class mate that I rarely ever spoke to, I would've deleted you from my friends list. It wouldn't have been because I don't like "you" for being the person that you are, it's just that I don't hold you dear. Teachers and... acquaintances... I don't really hold them dear... if you know what I mean.