(Native American English speaker here.)
Writing you're instead of you are is actually following standard written English, not merely reproducing the sound of speech.
On most on-line forums in English, the usual custom is to follow the standard conventions for written English, in an informal register—not to write carelessly or sloppily, and not to attempt to accurately convey the sound of casual speech. There is still a distinction between written and spoken language. For an illustration of the negative reaction to people violating the conventions of written English on an on-line forum, see here.
Written English, even at a high level of formality, includes a set of standard contractions, though not all the contractions that people use in casual speech. I don't have a complete list, but aren't, don't, hasn't, haven't, won't, can't, shouldn't, who's seem to me the most suited for formal writing; I'm, you're, he's, she's, it's, there's, I'll, you'll, he'll, she'll only slightly informal; it'll, there'll, I'd, you'd, he'd, she'd, it'd, should've, would've less formal but still part of the written language. C'mon is right at the border: a written convention that exists only in very informal writing. Here are a few that occur frequently in speech but are normally excluded from writing: ain't, gonna, shoulda, 'nother, and omitting the final g from any present participle, as in goin'.
Many people do write sloppily on on-line forums, but I think that's mostly because of carelessness, hurry, and/or ignorance of written conventions, not because of a deliberate attempt to adhere to a convention of reproducing casual speech accurately. It actually takes care, art, and knowledge of written conventions, like the use of the apostrophe, to reproduce speech accurately in writing. So, you're more likely to see your or youre in careless writing where standard written English would call for you're. Another example is writing should of for should've; that results from not even understanding that it's a contraction.
Here's one more example: writing tetnus for tetanus is unconventional even though it's occasionally done on-line and it faithfully reflects some people's speech. A writer wanting to reproduce speech would write tet'nus or tet'n'us depending on how many syllables there were. Writing tetnus suggests that you don't know the spelling and are trying to spell it as you pronounce it. (It could be a typo, but there's a strong pressure in American speech to reduce the second syllable.) So, writing tetnus violates written convention and therefore comes across as careless or ignorant. Writing tet'nus to convey casual pronunciation follows written convention but would be extremely unusual in an on-line forum because we don't normally try to convey the sound of casual speech in writing. Normally you would write tetanus even if you pronounce it with two syllables.