Why did people suddenly adopt a symbol for clearly marking their irony when before they have not?

My hypothesis is that prior to the mass adoption of mobile phones (SMS) and the Internet (IM, e-mail, any form of chat and discussion) it was nearly impossible (well, unpractical) to have informal conversations with someone in written form. Even letters or postcards are very much unlike e-mail, a lot more formal, for the most part carefully crafted and lacking the immediacy of our new forms of written communication. We used speech for all our informal conversations up until very recently.

Everyone knows how to mark speech as ironic. You can literally wink, change the tone of your voice or just start grinning after everyone starts looking at you with disbelief. When authors wanted to use irony in their works they had the time, space and skills to create a setup which marks something as irony, a typographic sign would probably have seemed like cheating to them (much like overuse of the exclamation mark would).

But suddenly there are all these people who are not authors and who write all the time if they want to have a conversation. Time, space and skills – the three of those are not readily available when chatting. (And why should they, why should you suddenly invest more time to conduct your informal conversation just because it is in written form?)

I think it works quite well, actually, for what it is, compared to its cousin in spoken informal conversations at least. Sure, it’s not even the same ballpark as some of the irony you can find in literary masterpieces but hardly anything we say in everyday conversations ever is.

(For testing my hypothesis I would suggest finding written pre-Internet conversations – if the is at all possible – and looking for any marks for irony or tone of voice. Pretty much seems like an impossible thing to check, if you ask me.)