In case you haven't seen them, there are a couple different threads already on the subject:
https://www.shapeways.com/forum/index.php?t=msg&&th=1 1752&goto=57574#msg_57574
https://www.shapeways.com/forum/index.php?t=msg&&th=7 426&goto=40282#msg_40282
I can see points on both sides of the question.
I recently discovered a mistake in one of my models, and I asked
service@shapeways.com to notify the three people who had previously purchased the items to let them know how bad I felt about the mistake, and that the model had been corrected. (to some buyers, accuracy is everything) CS passed it on to them and I still don't know who the buyers were - that's fine with me.
For my own two cents, it's a matter of privacy with me. The LAST thing that I am interested in would be for someone to go on a hard-core marketing campaign, spamming me with "Buy this NEW item from me!" emails every few hours. On every designers page, there is a "contact me" button. If I want to talk to you, I'll use that button. And.. if I am purchasing some necklace as a surprise for my wife, I very well might not want that purchase to be reported up on Facebook. Once Shapeways gives out my contact information to a third party, there's nothing I can do to get it back. I don't want that.