When to use ignorant stand definitions

When to use “at” and when to use “on” [duplicate]

Which of the examples is correct:

welcome at StackExchange

or

welcome on StackExchange

Or are both allowed?

UPDATE

I'm currently translating a website into 3 languages ​​and I'm a little confused.

I think that fits in better, although, as @One has discussed, both are acceptablewhich is the cause of my slight confusion.

On the internet you can endless Find examples such as:

welcome at Facebook ...

or

welcome on Facebook ...

and for example (original from the German Facebook page)

You own all content and information that you on Facebook post.

or

...If you at Posted on Facebook.

You could also use the last example on to write:

...If you on Posted on Facebook.