https://en.wikipedia.org/wiki/AOL_search_data_leak
http://techcrunch.com/2006/08/06/aol-proudly-releases-massiv...
http://www.nytimes.com/2006/08/09/technology/09aol.html?page...
TL;DR: It's fairly easy to deanonymize datasets like this, provided they are somewhat complete.
We already learned that Warhol's 15 minutes of fame should read 15 megabytes , but, to cut the "it's the users choice to post that data" apologists short: almost no-one I speak to understands the implications of all possible interpretations, classifications and groupings that their online traces allow.
Giving it to 4sq for data mining is different than giving it to UMN and/or the whole internet for data mining and/or deanonymization.