How should I phrase it:
This question has been asked at Stack Overflow.
This question has been asked on Stack Overflow.
Prepositions are tricky. You can put something in a book or read something in a newspaper, but you always put something on the internet or find something on a certain website. Unfortunately, I don't think there is a general rule that can be applied here, but when it comes to the internet and websites, "on" is the most common preposition.
The only time you might see "at" used is when describing the full URL for a website:
"At" tends to be used with specific locations, i.e. when something is at the top of the page, or at the end of an article.
I don't think "at StackOverflow" is wrong; that is, I don't think you must always say "on [some website name]". At can be used for fairly broad locations, such as "at home". However, I can think of one connotation of at that I don't think occurs with on, which is the distinction between related things with the same name.
in the New York Times
That is, in the paper
on the New York Times
This only makes sense for the website
at the New York Times
This could be referring to the organization itself, e.g., the workplace, the company, etc. But if it's clear from the context that you are referring to the website, I don't think it's wrong to use "at website".
It seems to me that this is actually a question of In VS On
The use of prepositions, both in the written word as well as verbally, can be tricky to use. Even very experienced writers may find themselves misusing a preposition if they aren’t paying careful attention. When it comes to in vs. on, you may think the application is simple, but in certain situations, you may be misusing the word.
When using the on preposition in a sentence, there are various situations that might call for it. Typically, you use on whenever you want to denote the position for surfaces or a position just above or outside an area. For instance, whenever setting something on the surface of something: I set a pencil on my desk, or the cup is on the table.
You might also use on when referring to a device or machine. When someone is on the phone, for example, this indicates a person is using the phone. Although this is an idiomatic phrase that makes little sense when taken literally, on is the correct preposition in this instance. Likewise, someone could also be “on the computer” or “on TV” in the event that someone you recognized was being seen on your TV screen. This wouldn’t mean that they were physically sitting on the TV, and 'in' wouldn’t be the right preposition in this case either, as that would imply they were actually inside the television.
On could also be a reference to a part of the body. As an example, you may might say a flung rock struck you on the shoulder, or that you witnessed someone slap someone on the cheek. When someone gets engaged, she wears a ring on her finger.
Another usage for on could involve the state of something. If a building bursts into flames, you would describe it as being “on fire.” Likewise, if you’re out shopping for clothes and there is an item that is 50% off, you might say that item is “on sale.”
When using 'in', you’re typically talking about something contained in an object, or something that is inside. Usually it references something that is in a position with space limitations.
It may also be used to represent general times of the day, month, year, or season. For instance, if you say that you tend to wake up early in the morning, or that you work less in the summer.
In can work when referring to a location or place as well. If you’re travelling, you might be staying in a hotel, or if you’re at home, your residence may be in New York. This location does not necessarily have to be a physical place, however. If someone shouts loudly in your ear, or if you are staring directly in the eyes, these are also places that the in preposition works for.
It’s also correct to use in when discussing shape, color or size. When looking at a painting at a museum, you might say that it was done mostly in yellow. Perhaps a group of spectators gathered around an unusual event and formed a ring around the perimeter. This would be the spectators gathering in a circle. When it comes to size, think of clothing. If you’re describing whether a shirt is available in large or medium, you’ll need to rely on the in preposition.
Lastly, in could also be used to describe a belief, opinion, or interest. For instance, you might believe in donating to charities on a regular basis, or have no interest in politics.
So based on this information, it seems to me, that it should be "question on Stack Overflow", and this is the same view that Grammmarly (.com) grammar experts hold. If you take Stack Overflow to be literally what it is, that is a Q&A platform, then you realise that our view is correct. Grammarly compares In and On using Facebook, which is a Social Media platform.
I hope that I have answered your question: The answer is on.