You can certainly see examples of major dialects like Cantonese/廣東話: http://weshare.hk/pandoracfrance/articles/1116298. I think there is a special IME for entering characters specific to Cantonese, to answer your question about typing it in their dialect. I would think Taiwanese (or 閩南話) would have some web presence as well but don't know for sure. Consider looking in non-obvious places if you're interested in more examples: frequency of dialectical usage in Chinese porn, for examples, is noticeably higher than in more "acceptable" contexts.
Part of the difficulty in answering this question is that, from my non-professional-linguist perspective, written forms for many Chinese dialects seem so close to written "standard" Chinese that it's hard to tell them apart, when reading, from variations within "standard" written Mandarin, with maybe the exception of the appearance of some more classical particles (ex 莫 instead of 不要, 汝 instead of 你) in contexts where they wouldn't ordinarily show up in Mandarin. Also most presence of dialectical phrases in written Chinese on the Internet, that I've seen, seem to be sprinkled throughout standard Mandarin.
The idea of 網絡語 (Internet speak) ref'd by yorelog being a dialect is interesting! Any trained linguist willing to discuss that?