One, two or even three exclamation marks are often added, especially in e-mail, to convey emphasis to phrases such as Thanks!, or No problem!. My problem is that in British English, you could also use the exclamation mark to convey sarcasm such that Thanks! could just as well be interpreted as Thanks a bunch, why on earth did you have to do/say that!. Is this just a simple regional difference? I make a point of suggesting workmates use the exclamation mark with great caution! (emphasis intended)
Given how bad direct text is at transmitting nuances of expression such as sarcasm, I would definitely have to chalk this up as a regional thing. Certainly in most of the online communities I have participated in, the expression of sarcasm uses a variety of fairly clear indicators:
Oh Noooooooooo, this isn't sarcastic at aaaaaaaaaaall.
I'm so glad you're here! </sarcasm>
I'm not convinced that in British English an exclamation mark does indicate sarcasm, in any case it totally doesn't work on me.
As Hellion hints, text is no good for this kind of thing, especially not if it's 'encoded' in some supposedly regional usage. So I'd either go for something explicit and unmistakable, or leave it out altogether.
Apparently there is a punctuation for Irony & Sarcasm:
American English doesn't typically denote sarcasm with punctuation. "Thanks!" is genuine. But emphasis or additional words can make it sarcastic:
- Gee, thanks.
- Thanks a lot.
I've also seen younger generations (born 1990+) use trailing letters to denote additional emotion that is not necessarily sarcastic: "Thanksssssssssssss!" I don't get it.