How is amount of water on earth always constant when so much of it is used by humans?

No water is used by anything.  Nor oxygen.  Nor carbon.  You drink water and you urinate which goes through treatment and is put into rivers that flow to the sea where the sun evaporates it. It forms clouds.  The clouds produce rain which goes into rivers and dams and finally through pipes so you can drink it.

Every single drop of that 100 gallons eventually finds its way back into the clouds where the cycle recommences.

The same is true of carbon.  We are increasing the CO2 in the atmosphere at the moment.  But that is because hundreds of millions of years ago plants used the CO2 forming carboniferous deposits and oxygen.  LOWERING the CO2 alarmingly.  We are currently rectifying THAT environmental mistake returning the carbon to the atmosphere from whence it came.  It might kill all humans but the ENVIRONMENT would have completed a full cycle and be back to where it once was.

When you flush the toilet the water does not vanish from the earth.

strangely enough the amount of water we use and the overall weight of the planet cant vary ,water is a commodity , it evaporates but does vary , most the food we eat has a water content , bananas are 70% water, same as ship might weight 60,000 tones but its only soil that has been converted any way .

Ours go into the septic tank and then from there, into field lines and evaporated out from the ground and grass back into the atmosphere. Water can't be destroyed. It just goes from one state of existence into another.

I drink a pint of water.  it passes through my digestive system and gets processed.  Some is retained until it can be used.  Most is passed out through urination.  That water goes through processing before being pumped back into river or lake from which it came.  The water is pumped through a processing plant and the cycle continues.

Next time you drink a glass of water remember, that same water once (or several times) passed through the digestive system of a tyrannosaurus rex.

Because law of conservation of matter.

Humans use it, but that water they used showering and flushing ends up in the drain and then back into the water system eventually.

When you drink water, you urinate.  That water goes back into the water system.  The water is not gone.

the earth is huge, and mankind is trivial.  Local balances can be offset, but the global balance is pretty much beyond our current ability to affect.

You are not recognizing the HUGE amounts of water that most places have to deal with constantly.  The dang St-Lawrence River passes more water in 100 seconds (on average) than gets used by all humans in Canada in a day.  And Canada has a very high per capita water consumption, to put things in perspective.

Besides, most of the water that gets used is put right back where it came from.  No net loss. Just a short diversion. In a regional sense, the only actual loss is from evaporation. Everything else ends up back more or less where we took it from.  The 10 liters or whatever used to flush a toilet does not miraculously disappear from reality.  It just takes a long route back to the river.

Now, if you are talking about Nestle or whoever pumping the crap out of an aquifer to fill bottles with water that gets shipped away, that is a net loss, and could be a problem if the system is very dry to begin with (has low recharge rates), which is why we normally would not let nestle or whoever pump water in such a situation.

Have you never learned the cycle of water, or are you 10?

The water does not disappear.  It goes into the ground, filters through the ground, goes into rivers and streams, goes into larger bodies of water, all the while it is also evaporating, becoming clouds, and raining down again.  Plants and animals take it in, and sweat it out. 

You use the water, then the water cycle cleans it for reuse. 

Where do you think that water goes when we use it? It doesn't vanish, it's still here on Earth. Did you pay attention in elementary school science class when they covered the water cycle?