Back during Google I/O, Google's Matt Cutts tweeted that Google "took action on several thousand linksellers in a paid-link-that-passes-PageRank network".
Cutts didn't provide much in the way of detail, other than also saying, "It's safe to assume webspam will continue to tackle link networks that violate our guidelines".
Last week, Cutts tweeted, indicating that it was about Text Link Ads link sellers.
@mikrutd you might have noticed a big drop in PageRank on thousands of TLA linkselling sites a couple weeks ago.
— Matt Cutts (@mattcutts) May 29, 2013
Cutts further clarified over the weekend:
— Matt Cutts (@mattcutts) June 1, 2013
Google has further indicated in recent weeks that it is cracking down on paid links and advertorials even more. If you haven't seen it yet, watch Cutts discuss this here.[h/t: Search Engine Roundtable]