Helium is lighter than air, so it should fly off from Earth. Is it possible that in the future we will run out helium?
Yes, helium can leave the Earth, and yes, we will run out of helium, but because of different reasons.
When you buy a helium balloon and its contents get released, this helium goes into the atmosphere. It isn't gone, and it could in principle be purified out of normal air. However, the total amount of helium in the atmosphere is so small it is technologically not feasible to reclaim it. At some point the technology might be developed, but it is unlikely to be economical.
On top of that, helium does also escape from the atmosphere. Since it is so light, it drifts naturally to the upper layers, and there it is easily torn away by the solar wind. However, this process will occur on geological timescales, unless we were to waste so much helium that the total atmospheric content changed appreciably. Keep in mind, though, that even if the helium doesn't leave Earth it is lost to us once it's diluted in the atmosphere.
So: yes, we will run out, and yes, it will make everything awful. And yes, you should cringe when you see helium balloons at a childrens' party.
As we know Helium is lighter than air, so basically Helium fly off from Earth
Just because Helium is lighter than air doesn't mean that it flies off Earth. At http://en.wikipedia.org/wiki/Atmosphere_of_Earth you can read that
[In the Homosphere] ... the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.
This has the consequence that Helium is equally likely to be found at the surface of the earth as at the top of the homosphere (up to ~100km). The ratio of Helium here is 5.24 ppmv (from Wikipedia page).
The fact that a helium balloon rises is due to the fact that the helium is kept together. If you would just release a lot of helium into air, it would spread out and mix, and not rise indefinitely as one might expect.
In the heterosphere (>~100km), however, the pressure is low enough to allow the gases to stratify by molecular weight (related to the Mean free path, see the Wikipedia page). So if you would release helium above 100km, it would rise above other gases, even without a balloon, which is why helium and hydrogen tend to be found further out in the hetereosphere.
Short answer: yes.
Given the value of helium, we should be making every effort to conserve it. When we run out of fossil hydrocarbon fuels we can find alternative sources of energy, but when helium is gone, it's gone.
I cringe every time I see a kid's birthday party with helium filled balloons.
It would probably hang around just around the atmosphere, because even though being lighter than air, it still has mass.
Also, helium is created on earth from radioactive decay in elements like uranium, because a helium nucleus is a deadly alpha particle. As long as we can make alpha particles, we can make helium.
Helium is also created in stars during nuclear fusion.
Since Helium is so light it can actually escape into outer space. See here. Most of the helium we use comes from radioactive particles decaying underground. Yes, we can run out, and yes, it will make everything awful.