Looks like there was a small glitch for May 6th in regards to ACRU/CRU tracking. Here is what they had to say about it..
There was an issue that started at 5pm on Wednesday May 6th with the system that assigns CRUs and ACRUs to individual accounts. Engineers have now resolved the issue, unhealthy but you may see some very low numbers or even zero for your CRUs and ACRUs in your reports for the dates of May 6th and 7th. The underlying new user events are all tracking correctly, click so we have the data we need to credit the right accounts, and calculate the appropriate account level quality scores for the US program. All affected publishers will be compensated through a bonus in their next payment, although the missing transactions will not appear in the reports.