It had been reported recently by various outlets that NXT: Takeover Brooklyn 2 had higher viewership numbers than WWE SummerSlam, which aired the following night. It appears that is not actually the case.
The information that was used to determine the initial numbers was taken from Video on Demand, and not from live streaming. Also, SummerSlam could be ordered on pay-per-view, which likely added at least tens of thousands of extra viewers, while Takeover was only available on the WWE Network.
It did seem a bit odd in the initial reporting that more people watched Takeover. While most fans would argue it was the far superior show, the brand is still only watched by a fraction of WWE's overall audience.
Raw and SmackDown combined have five hours of primetime TV a week to promote SummerSlam, while NXT only has one hour a week on the network, meaning potentially millions of more people watch shows based off the main brands. This is why Talking Smack actually outdrew the NXT special.
Here is a more accurate ranking for most watched shows on the network last week:
5. NXT Takeover Brooklyn II4. WWE Cruiserweight Classic (S1 E07)3. NXT TV (Episode #351)2. Talking Smack1. WWE Summerslam 2016