Microsoft is now rolling out the Windows 10 9879 preview build to users who have selected to be on the Slow ring in terms of getting downloads via Windows Update. Surprisingly, it appears that's the most popular way to get these preview builds.
Microsoft sets the default for Windows 10 updates to "Slow" but users have the option to get them quicker if they simply select to be on the "Fast" ring update list. However, not many folks make that switch. In a Twitter message today, Microsoft's Gabriel Aul commented, "Fast ring is about 10% the size of Slow ring. This is exactly what we hoped to see."
It would appear that most Windows 10 preview users are simply choosing to stick to the default "Slow" ring rather than get on the "Fast" ring, which provides earlier updates but which can also have bugs that have to be fixed for Slow ring users. Which ring are you on?