[Zairak]

Speaking for the general population, this is a simple cultural phenomenon. Just as 'dude' and 'gay' and other such words come to be used in different ways than originally intended, so too do phrases such as 'Oh my God'. I feel it's safe to say that the phrase as it is used on the internet is generally one of surprise, whether sarcastic or as an honest expression.

On a heavily related note, it is important that 'Oh my God' is so easily shortened to text speak. This has facilitated its spread through various chat clients until the present day, when it is now a stock phrase for, again, surprise, whether sarcastic or literal. It may be that it lacks religious significance to most people for this very reason. It's very simple for acronyms themselves to be disassociated from their original meanings, and when phrases are used several times a day, as happens now with 'omg', the word or phrase becomes much more casual. It simply can't help but do so by its repetition. It loses power and its original relevance.

That being said, it likely has a fairly wide range of variance in definition for individual people, and outside the general meaning I mentioned earlier, 'surprise', it's pointless to try to categorize a certain group of people's use of the word. It's become far too casual of a phrase to be anything but highly individualized and, therefore, not specifically definable.

I might add that this is similar to some people's treatment of the word 'religion', but I don't care to delve into that specific topic outside this snippet.