HOW facebook / Instagram for administrator contact

Facebook won't answer you - at least, a human won't. I guarantee this. They have over a billion regular users and fewer than 26,000 employees of all sorts (that includes the janitorial staff). No one has the time unless you are a BIG celebrity.

So if you're not Lady Gaga, then you can forget about getting a person at FB to answer you. Your letter will not be read by human eyes. A phone call will not go any better. Don't call them.

What are you trying to do? Speak to the person "in charge"? Then you can forget it. You're not going to get a meeting with Mark Zuckerberg this way.

Are you trying to get back administrator rights to an account? Then go looking for how to do that in their help centers. Recognize that even if you follow their instructions to the letter, you still might not get effective help.

It's not that they don't care (well, mostly it's not). Rather, it's that they don't have enough bodies to throw at such problems.

If you are trying to get back administrator rights to an account, unless it's got a lot of followers (like on a page or in a group), or you're being impersonated, or you have valuable data on there which is only there (try not to ever have this be the case), then the more productive and more likely to work solution is to just make another account.

I'm just a facebook user,that is, everything is useless !!! the letter was useless!!!
tknowing that there are millions of users etc ..

Since I cannot tell you what or what not to do, nor encourage or condone this reply (So I am not responsible for what you choose to do), but if it were me,
I personally would just go to Marc Zuckerburg's house or wait outside the corporate office rationally, calmly and peacefully, until everyone starts leaving for the day. Then just say hey would you please read this. If they don't, then nothing you can do, I would just leave without incident with the thought, least I tried.

That is just me and what I would do.