Online advertising companies have struggled for several years as Google and Facebook solidified their grip on digital dollars, slowing revenue for the others.Now, many ad tech companies and their investors are throwing up their hands.

Venture capital money going into ad tech start-ups is falling sharply, helping push a wave of consolidation. Financing reached a high of $2.92 billion in 2015, but this year, it is on pace to be less than half that, according to CB Insights, a financial research firm.The number of independent ad tech companies has fallen 21 percent since 2013, to 185 as of the second quarter of 2018, according to LUMA Partners, which analyzes digital media and marketing.

And the pace of contraction has been quickening considerably. This summer has seen a flurry of activity, with AT&T buying the online ad exchange company AppNexus in June, and the private equity firm Vista Equity Partners acquiring a majority stake in Integral Ad Science the same month. In early July, the Interpublic Group acquired the data aggregator Acxiom’s marketing division for $2.3 billion.

Although many consumers have never heard of ad tech firms, people’s online activity is influenced every day by these companies as they battle for a share of ad impressions on phones, tablets and laptops. The “Mad Men” style of advertising workers has been replaced by the “math men” of ad tech start-ups, which specialize in gathering data on consumer preferences. 

Advertising, the economic juice behind the internet, has long been an attractive area for start-ups. During the last 10 years, the ease of forming companies and the availability of cheap venture capital led to a flood of ad tech start-ups, pushing boundaries on where and how ads were delivered. They introduced technologies like the automation of ad buying — called programmatic advertising — and header bidding, in which many ad exchanges bid on publishers’ space simultaneously. Overall spending for online ads continues to rise, to more than $88 billion last year, according to the Interactive Advertising Bureau, a trade group. But more than 90 percent of that growth last year went to Google or Facebook.