When it comes to the vast sector of companies focused on buying and keeping customers, sales tech is the most interesting these days for an investor — followed by marketing and, least of all, ad tech.

That’s the word from Byron Deeter, a partner at VC firm Bessemer Venture Partners (BVP).

Previously, he’s been the founding CEO of Trigo Technologies, which offered product information management, and then transitioned into an IBM exec when Big Blue bought Trigo in 2004. I checked in with him recently to get an investor’s perspective on the vastly complicated and overpopulated space of sales, marketing, and ad tech companies.

Hovering over all sales tech companies — which go by such labels as sales accelerators, enablers, forecasters, and opportunity managers — is the 800-pound gorilla known as Salesforce.

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They’ve done well, Deeter acknowledged, having grown into the market leader in cloud technology.

“But,” he added, “they’re a 15-year-old platform.”

And that age, plus their inability to “innovate fast enough,” equals market opportunity, he said.

“Salesforce has validated that there’s a mass market, [with] unlimited market size,” he said. “But there are opportunities to extend Salesforce because of the new directions in social and mobile, [and] they’re not able to innovate fast enough.”

As others have, Deeter pointed out that the best-positioned big challenger to Salesforce is business network LinkedIn, which is busily launching sales and marketing cloud solutions to take advantage of its extraordinary, massive, and detailed professional membership.

“It’s the most updated company record,” he said, since bringing your LinkedIn profile up-to-date is the first thing people do when they change jobs.

But there is still room for companies to challenge or expand on Salesforce. He pointed particularly to teleconferencing app provider Speakeasy, a part of Bessemer Venture’s portfolio and a solution that extends Salesforce. Another BVP example: customer retention provider Gainsight, which is integrated with Salesforce.

Marketing tech companies may not have the innovation opportunities that sales tech firms do, but he noted they can “become stickier with success.” A martech solution integrates with a customer company’s other tools, and, if it succeeds, you can build an entrenched user base.

One issue — and growth opportunity — for marketing automation companies has been that the technology is implemented in less than five percent of firms, with a heavy concentration on tech companies.

Deeter noted that this figure doesn’t fully account for email, which most companies are using as their central marketing tool. But, while there may be growth opportunities for marketing tech companies to push past five percent, the level of innovation has tapered off.

“The first wave of martech has matured,” he said, with such mainstays as Eloqua, Marketo, and ExactTarget covering a lot of software categories.

There’s a “little bit of buying fatigue,” he said, as investors look for the next stage in evolution. For both sales and marketing tech companies, he said, using big data in a mobile context offers some possibilities to leapfrog the legacy companies.

As for ad tech companies, customer stickiness and differentiation are both problematic.

They don’t need to integrate with many systems within a customer company, he pointed out. If they deliver what you want, you simply buy more services from them — or from other parts of the digital advertising ecosystem.

There are hundreds of ad tech companies, he noted, and “a lot have built cash flow, [but] not enterprise value.”

“Ad tech is all about acquiring traffic for less than what you sell it for, efficiently,” said Deeter.

“The middleman can profit, and profit beautifully. Google has this down. Criteo does this with display,” he added. “The vast majorities will create cash flow from mobile, but ultimately will sell for low single digits [times their] cash flow.”

While there are “massive incentives for publishers and ad tech to goose the numbers” with traffic fraud and other manipulation, he said, “it will catch up with you over time.”

If all you have to sell is “crappy traffic,” Deeter said, “buyers will stop buying from you.”

“At the end of the day, you care about [customer] acquisition.”