Part of the issue using this Website's traffic as a metric to how a WoW Twink site is doing is that Xpoff was (mostly) created for the 19 Community to get away from everything that wasn't 19's on TI. The majority of Twinking Players that participate in the TC are Level 19's. The majority of Level 19's H-A-T-E Legion's re-merging of Xpoff/Xpon Bg's, thus aren't playing nearly as active as they once were.
What would be more interesting to find out is how many NEW, non-old school 19's, have come to the Site since Legion's launch. That would give us a better understanding as to whether this site lives (and/or dies) by 19's alone or if the entire Community is what keeps it going, even in spite of 19's inactivity.
EDIT: Just re-looked at the graph, It follows almost exactly new Content Patches and Legion's launch, just like Sub numbers for WoW itself. I'd wager you'll see another spike shortly as the new DMF section opens as Twinks are dying to get their hands on that sweet Mace.