The major indexes may have been flat during the month of August, but there was a lot of buying under the surface, especially in popular stocks like Facebook (FB) and Ford (F) , says Joseph Kinahan, chief strategist TD Ameritrade (AMTD) .

"Retail investors didn't short or sell the dull market in August, they bought things they knew," says Kinahan.

TD Ameritrade's Investor Movement Index (IMX) rose 12.1% in August to 5.26 compared to an essentially flat month for the S&P 500. The jump was the largest single month increase for the index. The 52-week high for the IMX is 5.39 while low is the current 4.33.

Each month, TD Ameritrade pulls a sample from its client base of 6 million funded accounts that includes all accounts that completed a trade in the past month. The holdings and positions of this statistically significant sample are evaluated to calculate individual scores, and the median of those scores represents the monthly IMX.

Despite being net sellers overall in August, investors were net buyers of some popular names. Facebook set a new all-time high for the second month in a row, and continued to be a net buy for TD Ameritrade clients.

TD Ameritrade clients appeared to see some opportunity in dividend paying issuers who have seen recent declines as well. Ford, which has been a popular net buy for the last few months and yields 4.8%, and Pfizer (PFE) , which yields nearly 3.5%, were net bought in client accounts.

Earnings announcements also continued to gather some attention into August. In the technology sector, Nvidia (NVDA) was a net buy up to and following its earnings report. Additional popular names bought included Bristol-Myers Squibb (BMY) and Wells Fargo  (WFC) .

On the flip side, Bank of America (BAC) was net sold as the company's share price was up nearly 9% at the close of the August IMX period. Alibaba (BABA) , which reported earnings in August and saw gains following the announcement, was net sold as well. Microsoft (MSFT) set a new all-time high in August and Yahoo! (YHOO) reached a year-to-date high. Both were net sold in TD Ameritrade client accounts. Additional popular names sold included Alphabet (GOOG) , Citigroup (C)   and Caterpillar (CAT) .