I agree with both of the above posts, and I would add one other aspect to the discussion. Abigail and the other girls begin this frenzy with one simple goal in mind--to avoid punishment. Some of the girls seize the opportunity to exact revenge on those whom they believe have wronged them, and Abigail sees this as a chance to eliminate her competition for John. Salem is perfectly positioned for this particular diversionary tactic, so it works. Today it kind of looks like the "blame game," in which certain types of people want to blame others for their problems in hopes of avoiding taking any blame for themselves.
Witch hunts happen when we face problems that we do not understand and/or problems which seem to threaten our very way of life. In such situations, we try to find scapegoats to blame for our problems.
A liberal might say that an example of thisis the way that we look at illegal immigrants today. We know that our economy is struggling and that American society is becoming less like it was in the old days. We do not know why this is happening, so we blame it on illegal immigrants.
A conservative might make the same argument about rich people. They might say that we blame the rich (bankers, Wall Street, etc) for our problems because we don't understand them.
None of these things has reached the level of witch hunt yet, but the idea is the same -- when we encounter problems that scare us, we look for scapegoats. We generally pick people who are unpopular and use them as our scapegoats. This can become the basis for a witch hunt.
We generally project our worst fears on groups of people that we perceive as completely "other" from us and who we think are working to subvert the established order. These fears might not be wholly justified by facts, but may be the result of the ways these groups are depicted by the media or the ways they are perceived because of some more outspoken and more radical members. Fears of conspiracies have constantly hunted American history.
With the end of the Cold War, Communists no longer posed threats to the United States. However, the events of September 11 identified a new enemy, Muslim fundamentalists. The fear of terrorist attacks has led to the approval of measures such as the Patriot Act (2001) which had restrictive effects on civil liberties. The depiction of fundamentalism as integral to Islam has provoked a wave of suspicion and even violence against Muslims embodied in the tortures in Guantanamo and Terry Jones's Koran burning.