Hi Tony,
I would recommend looking at the Website Traffic Analysis Dashboard vs. the Social Media Breakdown report. On the Website Traffic Analysis Dashboard there is a tab for "Top 10 Source Domains", in that area there are sub-tabs for referring traffic by Email, Search, Social and Website. These are essentially filters for those specific types of traffic. If you don't see an expected domain on the Search or Social tab, you can click over to Website and you will see the full list (clicking the "Detailed Data" link at the bottom will show more than the top 5 or 10 results). My understanding is that the Social Media Breakdown report is essentially the same results as the Social tab in this dashboard. Our Product team tries to keep up and categorizes websites by Search or Social so they show up in these filtered reports, but it looks like Pinterest hasn't been categorized.
Let me see if I can have someone from our Product team weigh in as well. I hope this is helpful in the meantime.