Roger Clarke, who wears several hats as an academic and consultant in Australia and Hong Kong, focuses not on the privacy of those who sign up for such services but “on a matter that is new, and of great concern: the privacy of other individuals whose data is volunteered to such services by its users.”
The piece is worth reading. He makes some important points about how this is more than just an issue of some sleazy marketing guy making use of your data to sell you stuff, or build a profile of your shopping habits. He also points out that this kind of data — stored by individuals in a private capacity — is not covered by most data protection laws.
His conclusion: “In general, people would be well-advised firstly to stay well clear of all address-book and ‘social networking systems’, and secondly to prevail upon their friends, colleagues and acquaintances that they should avoid making any data about them available to service-operators like Plaxo.”