If you are a COD fan or just want to play the active one,get WiiU and Black ops 2 (if you can afford it)
But to answer your questions:
About 10 months with no patches.
hackers are there,they do bad things and ruin the match,but there are still good matches.
Everyday about 3000 people play MW3 on wii.
I haven't seen a huge amount of hackers, although I do find a (obvious) hacker usually at least every 5 or so matches. If you think about it from the hackers' point of view:
1. None/Very little regulation of MW3 Wii.
2. Hey, I've bought SD card(s) for hacking my Wii, why not get Wii U and BO2 to play the coolest/ latest CoD games?
Overall, YES there are more hackers on MW3 Wii due to no/very little regulation. But at the same time NO, due to the release of Wii U and BO2. In other words, it's most likely about the same.
Some Advice: Buy a Wii U (probably has a LITTLE bit more "Anti-Hacker" technology installed into it (a.k.a. less hacker PROBABLY). Or you can buy an XBOX 360 which is pretty much as good as a Wii U.