Betterment has $8B as of March 16, 2017: SEC form ADV page 8, "Regulatory Assets under Management"
https://www.adviserinfo.sec.gov/IAPD/content/ViewForm/crd_ia...
edit: Wealthfront $5B https://adviserinfo.sec.gov/IAPD/content/ViewForm/crd_iapd_s...
https://www.adviserinfo.sec.gov/IAPD/content/ViewForm/crd_ia...
edit: Wealthfront $5B https://adviserinfo.sec.gov/IAPD/content/ViewForm/crd_iapd_s...