After years of tweaking anti-spam filters on my personal email server, I have all but banished Nigerian dictators ads for “viagkra” from my mailbox. But almost every week I find dozens of emails, allegedly from various friends and business colleagues, exhorting me to join every new social networking site under the sun.

As if the thicket of companies out there trying to build the next MySpace or Facebook weren’t annoying enough, each new venture seems to have gotten even more aggressive than the next in making its users crack open their email address book and launch invitations to everybody they got business cards from at a cocktail party in 1997.

The earliest social networking sites learned the hard way – by being blocked as spam and reviled by would-be customers as pests – that aggressive viral marketing can cause explosive growth, but can also blow up in your face.

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