UPDATED TO FIX MY MATH ERROR:
I posted some statistics on references cited in recently issued patents. That raised some discussion in the comments regarding third-party prior art submissions. The chart below puts some numbers on the result that everyone likely expected–third party submissions are quite rare. Out of every 10,000 issued patents, only about 14 include prior art submissions from third parties.
One aspect to consider about this data is that it may be skewed because it only includes issued patents — We can expect that there are a percentage of cases where the third-party submission led the applicant to eventually abandon the patent application.
Although third-party submissions were permitted prior to the AIA, the AIA expanded the process substantially – although only with regard to applications filed after Sept 2012. Today, third parties can submit prior art to the PTO so long as it is both (1) within six months of publication and (2) before an office action rejecting a claim or a NOA. The submissions also require a fee of $180 for every 10 documents submitted ($90 for small entity).
The following are some recently issued patents that include third party submissions if you want to look through the file histories: 11,228,023; 11,225,200; 11,224,546; 11,224,456; 11,228,055; 11,221,486; 11,220,614; 11,219,722; 11,219,166.