Here’s the FSRankings ranking of robo-advisors based on how many accounts they manage. A single person could have multiple accounts (e.g. if you have an IRA and a college 529 savings plan), so there may be fewer actual customers for each robo-advisor.
Acorns continues to far outpace the pack. But it has a very low dollar amount per account relative to the next two robo-advisors, Betterment, and Wealthfront.
Note: Data sourced from the companies directly and SEC filings. Not all robo-advisors listed. As of 1/29/2018. Contact FSRankings if you are a robo-advisor and would like your data updated.