I am going to go with Vince purchasing Nitro, for a couple of reasons:
First, the overall wrestling audience was larger then, which means more people watching, which means more people that not only watch, but also are members of Wrestlezone. It stands to reason. Larger audience means a larger pool of potential Wrestlezone members willing to hop online and talk about it...
Secondly, in addition to the logical Wrestlezone membership size increase, while Nitro was getting poorer ratings at that time then they had been, overall, the final Nitro scored a 3.0, which means that even with it's poor ratings, it was still getting almost as many viewers as RAW does now. Now, combine that with the 4.7 Raw received, and that's a combined 7.7 Nielsen rating. A LOT of people were watching wrestling on March 26, 2001.
http://www.twnpnews.com/information/WCW/wcwnitro.shtml
http://www.2xzone.com/ratings/rawhistory.shtml
Thirdly, and this is the most important part, I think: Once we have figured out that the wrestling audience was much larger, which would have meant a much larger Wrestlezone membership, you also have to consider that Vince buying WCW (didn't he appear on the Nitro-tron or whatever it was called, so technically he did appear???) it affected TWO wrestling companies. It would have been something that would want to be talked about by both WWF and WCW fans. Most WWE fans today don't care about what TNA does, and a lot of TNA fans don't care what the WWE does. But, Vince buying WCW was very relevant to both fanbases, and would have gotten both fanbases talking about it. That would have increased the Wrestlezone traffic even more than just wrestling being overall more popular. Vince buying WCW was important to the entire wrestling world, not just the fans of one of the organizations.
And finally, there is a significant amount of discussion that would have been generated because of the wide variety of potential topics involved with one wrestling company taking over it's main rival. What happens to the WCW wrestlers? While Nitro be on the next week? Is Vince going to run them both? Is Shane really going to be in charge, and so on and so forth. There are hundreds of threads that could have been spawned.