I don't think the paper mag and website activity are directly linked. From my dealings with members enquiries people are mainly from two camps - PC users and non PC users.
There are still a lot of people in the Z scene who don't want to use PCs, and they miss the paper mag. The Mag was brilliant but cost too much and info was slow and late.
So if someone is not 'into' the internet then losing the mag is not going to temp them onto the site. There are a lot of members who just don't want to get involved in internet 'small-talk' but others love it.
I use the site for information mainly, before we had an active site I got some info from the mag and some from Local Meets, or a call to Mr F. I think perhaps the Local Meets have suffered because of the internet.
I can't quantify any of the above it's just my perception.
The ZClub enhances my pleasure of Z ownership and provides more driving opportunities e.g. Trackdays, Japfest, Le Mans Classic, Gurston trial day, Santa Pod (only done one day so far) etc.