The White House says that over the course of the day the White House webcast of the summit served 3.9M streams and peaked at 60k concurrent viewers. By contrast the State of the Union webcast served 1.3M streams and peaked of 85k.
Now, this is clearly not an apples to apples comparison. You can see the State of the Union address on every channel. So there's not much reason to watch it on the White House feed unless you're stuck at your computer without access to TV. And even then you can watch it streaming on a bunch of news websites. But that's still a pretty robust comparison.
It's certainly not a perfect measure. But over the years I've gotten a pretty good feel for how different kinds of political events bump traffic on TPM. And there was a much bigger bump than I would have expected for an event like this.
Let me know whether you've come across other metrics that give some sense of how many people tuned in to watch some of this.