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40% of social network users are receptive to opt-in advertising, according to market research firm InsightExpress.

Meanwhile, just 23% of users on Facebook, Myspace, Classmates.com, LinkedIn and Reunion.com felt randomly generated ads were acceptable. This was in marked contrast to CafeMom, Twitter and Flickr users — 43% of which said random ads were acceptable.

"Recognizing the rapid growth of social networks and social networking audiences, advertisers have focused on creative engagement and how to apply their brands within a new environment," stated VP/Group Director-Digital Media Measurement Drew Lipner of InsightExpress.

"The broad and exciting acceptance of social networking also reinforces the distinct need to develop targeted and relevant campaigns for this channel with the help of advanced measurement tools."

InsightExpress claims 43% of the online population has reported using a social network site. And a full 71% of them maintain profiles on at least two or more.

Facebook and MySpace are the primary gateway social networks — that is, the ones where users begin exploring social networking before potentially branching off into other sites. Of social networkers that have only one profile, 46% chose MySpace and 36% chose Facebook.

Those with two or three profiles were broken down thus: 78% are on MySpace, 71% on Facebook, 22% on Classmates.com and 11% are on LinkedIn.

And of those with four or more profiles, 92% are on Facebook, 89% are on MySpace, 60% have a Classmates.com, 40% use Reunion.com, 33% have a LinkedIn, 11% have Flickr, and 10% have Twitter.

Last month a Netpop survey found the typical social network users addresses 110 people per week and spends $101 online per month.