I see a few different reasons and ways people make comparisons to Hitler and/or Nazism in posts around here; each reason requires a different response.
One is simply literary hyperbole, (exaggeration, for those of you who missed class that day

). It is not intended to make an accurate comparison or to imply the writer really believes it is true, it is a means of expressing extreme feelings on whatever issue is being discussed by comparing it to the most heinous series of events they can think of (sure, there are some other examples in history that could be used, but this one is more current and still evokes real emotion more than other events that are relegated to "ancient" history rather than "20th Century" history). When somebody is using it in this context, I just ignore it. They are at that point discussing feelings rather than facts, and there's little point engaging them on that; I have to accept that their feelings are their own and can only take their word for it that they feel the way they tell me. There's no point in arguing emotions.
Another way it is used is as part of a "slippery slope" argument. Those posters are not using it to say something IS like Hitler or the Nazi regime, or is directly comparable to the holocaust, but are saying events we see happening now are similar to how Hitler acted at the very beginning of his journey to power, and are arguing that if we don't stop them now, they will necessarily lead in that same direction. Of course, we know the fallacy of the slippery slope argument, and can address it as such. A lot of things had to fall into place at the same time for Hitler to have gained the type of power he gained, and anyone arguing that it will happen again needs to demonstrate how ALL those events are likely to recur.
However, related to the slippery slope usage, some people do make comparisons that are accurate and reasonable. They are not saying we're headed toward a Nazi regime or that Bush is Hitler, but rather are pointing out some of the commonalities of one era in history to the current political climate. There is nothing wrong with analyzing past political strategies and who is using them and how effective (or ineffective) they are. Politicians have studied politics, and if there are effective methods of leadership they see various people have used, they will borrow from those. That a device is effective and that you've borrowed it does not imply you will use it to the same end, or with the same motivation, as someone else. When people use it in that way, the problem is more the knee-jerk reaction of the respondents who misinterpret the meaning of the statement, rather than with the post making that comparison.
And lastly, there is the scenario of those who really do argue from ignorance, who really are blindly parroting the words of others or who don't see the blatant differences in what happened then vs now. I question if they truly understand the depths of horror that were a part of the holocaust to really believe such comparisons are accurate. With those people, all you can attempt to do is educate them on the differences, and remind them of what Hitler's regime and the holocaust were really about and hope it sinks in.