I've read only about five of the so far nine pages of replies, so I hope I'm not repeating anyone's reply here.
That said, I think these are most likely the reasons for so many people disappearing:
-- Child runaways
-- "Deadbeat Dads"
-- People running away because they did something illegal or just want to start a new life
-- People being murdered and their bodies hidden or disposed of
-- Children being kidnapped and possibly killed by pedophiles
-- Small children and babies being kidnapped and kept by emotionally disturbed, childless people as their own children
-- Children en masse being kidnapped by child prostitution rings (especially foster children, and there are stories that those rings go up to the
highest levels of government and corporate wealth)
-- Women being kidnapped by sex trafficking rings
-- People in witness protection programs
Plus, the weirder stuff:
-- People being abducted by aliens and never brought back
-- People disappearing through Spontaneous Human Combustion
-- People kidnapped for "black ops" government programs
-- People "falling" into another dimension
I think the "falling into another dimension" thing is really not as farfetched as it sounds. I believe there are two types of "dimensional tears"
or wormholes, whatever you want to call them: the fixed ones that stay in place -- like the Bermuda Triangle -- and smaller ones that move around or
suddenly open up and close again.
There's even good evidence for that theory: I myself know two people who've had something vanish in front of their eyes, never to be found again.
One is a friend of mine, let's call him Sean, who once was out in the desert rockhounding. He had half a watermelon and a spoon with which he scooped
out the melon flesh. At one point he dug in too deep and the spoon got catapulted out of his hand. Sean said he followed the trajectory of the spoon
with his eyes, and it felt almost like it was happening in slow motion. He saw the spoon flip end over end, really slowly, and fall into a small shrub
right in front of him. Now, this was a feeble little bush, and Sean searched it for the spoon -- for an hour -- to no avail. There were no holes
beneath the bush, the bush itself had almost no leaves (one of those desert evergreen things), and there was no way the spoon could have just
disappeared.
The other story came from my boss (who, I have to add, is a militant atheist and science disciple and scorns everything paranormal). He said once he
had a fight with his ex-wife (in the bathroom) and in frustration, threw a bottle of shampoo against the wall. It landed between the bathtub and the
toilet, and when he wanted to pick it up, it was gone. He searched the bathroom over and over, but it didn't show up again.
So, my theory is, maybe there are little holes in spacetime that suddenly open up somewhere and then close again. There's no reason why only THINGS
should be able to disappear in such holes, right? Why not a person, or a pet? That sure would explain a lot.
As to where the person goes: if the hole closes immediately after opening up, that would mean that the person has no way of getting back from whatever
dimension they "fell" into. And some of the dimensions may be so similar to our own that the person perhaps wouldn't immediately realize what
happened.
One poster asked "Then why aren't people from other dimensions coming here?" Well, maybe they do, but maybe they're not quite like us, if you know
what I mean.
What about apparitions like the Mothman, that show up for a few weeks or months, never to be seen again? What about the "black-eyed kids"? What
about shadow people? Maybe those are beings that have figured out how to actually find and USE those wormholes and jump back and forth between their
dimension and ours as they wish. Just a thought...