I use
Plaxo, a web-based electronic address book, to request updates on contact information from colleagues and friends, and to update them on any changes in my coordinates.

Plaxo is on my mind, because I recently used it to send an automatic update request to all of my contacts.  Many people didn't respond at all; many confirmed or modified the contact information that I had on file; a small number sent requests to be removed from my Plaxo file, or highly articulate jeremiads about Plaxo...
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