Probabily i'm the only user that for write a million write "1,000,000"...

Usually the other user write:

"1 million"

"1M"

"1000k"

ecc...

Any way, how you usually write the number's?

1k, 1 million, 100. I use all 3

Ks and Ms

Yes, but usually?

What is the writing mode used in this site for example?

Any form is technically correct, it all depends on how precise it needs to be. Generally I think the least amount of 0s is the best. But to answer your question, it really just depends on the context.

K for thousands.

M for millions.

B for billions.

This.

Depends on the number. If its in the thousands, I use k, if its in the millions, I use million and if its in the hundreds, I use 00

I always write the intere number's.

1  10  100  1,000  10,000  100,000  1,000,000  10,000,000  100,000,000  1,000,000,000

Ecc

100-58

That should solve every issue.

2,805,256/(280/5-56)

This one makes sure there are no issues.

But usually it's easier to abbreviate things with "k" or "m" or "b" or "t" when it gets into that range.