Ad hominem isn't used in earnest as often as the others mentioned, since most people aren't wholly convinced by it per se. One non-fallacious usage of ad hominem is to point out a potential conflict of interest. It can be convincing if used in a "he's a dirty [outgroup membership here]" way, to certain people.
Appeal to authority is used an average amount, often in quotation wars, but frequently devolves into an argument of which authority is more authoritative. Likely to lead to ad hominems against the authorities.
Appeals to emotion are probably the second-most-used fallacies on the Internet, due to the wide variety of emotions to appeal to and ways to appeal to them, some of which people can't even name ("think of the children!" etc.). They can be combined with nearly any other fallacy, as well.
Bandwagon isn't intentionally employed that often, since most people learn it's fallacious by adulthood. In any isolated groupthink environment it's likely to be implicitly in effect, perhaps with a "side with your friends rather than the outsider" angle.
Correlation vs. causation is only really in play when discussing statistical studies which don't involve controlled experiments; studies and even experiments are generally accompanied by appeals to authority ("ivy-league scientists say...") and bandwagon ("scientific consensus says...").
Slippery slope is likely the rarest, since it doesn't apply to historical discussions (if society slipped down the slope or not is a matter of fact) or binary decisions with no arbitrary cutoff. There has to be a slope to slip down, and many issues don't have one that would make sense.
I'm pegging Strawman arguments as the most common Internet fallacy (listed), by quite a bit, due to how easy it is to employ completely accidentally, even if one is wary of strawman arguments and tries to avoid them. Merely by failing to properly understand a position, one is likely to make a strawman argument in an attempt to clarify the issue under discussion. Trying to take a devil's advocate position for something you disagree with, without employing a strawman argument for it, is HARD. It's so easy to try to boil down the opposition to save time and mental energy, even if it loses the crux of their arguments, that one is likely to do so.
Perhaps, making my case for this, I'm strawmanning the other fallacies; even if true, wouldn't this prove my point? :P