Someone who is (or tries/works very hard to be) the most popular person on the forum or message board they frequent. Often, the Net Cack will attract other Net Cacks. It's easy to spot a Click of Net Cacks, because they often all stick together on every subject, attack people together - as a group, tend to have this energy of 'inside joke' in their posts, and/or are rude, obnoxious, stuck up, ignorant, and verbally abusive.
The Net Cack is usually quite unpopular in their real life, has very few real life friends, is hard to get a long with, and generally unhappy.
Often, the Net Cack is also morbidly overweight, and/or has a hideous appearance, leaving them with little opportunity to work their way up the social ladder in the real world. Thus, bringing them to the Internet, where everyone has the chance to be SOMEONE.
I was really enjoying that particular message board, until the blasted Net Cacks started taking over.