The data-driven marketing revolution is revolutionizing data companies.

As marketers tie digital data to loyalty, CRM and transactional data, firms like Epsilon and Merkle are treading on agency turf, buying or building ad technologies and digital-marketing capabilities, from ad targeting to mobile-app development.

"We are competing more and more with the professional-services firms," said David Williams, chairman-CEO of Merkle, who now considers companies like Accenture and IBM competitors.

Merkle's digital-services net revenue, including search, display, mobile-ad services and email marketing, grew 78% in the first half of 2013 year over year, he said.

Data and tech firm Epsilon, meanwhile, is branching into marketing services with the additions of Hyper Marketing (acquired November 2012), which offers social, mobile and retail agency services, and Aspen (acquired May 2011), which bolstered its email-marketing, mobile, creative and loyalty capabilities. Epsilon expects growth to ride on agency revenue. In second quarter, for example, its data business was flat year over year but its agency business more than doubled.

Another competitor, Acxiom, has pumped investment dollars into digital ad-targeting technology, and Experian is also offering agency services.

Clients may not be so quick to switch to data providers for agency services, warned Fatemeh Khatibloo, a senior analyst at Forrester Research who tracks the data-services industry. The firms might have trouble attracting creative talent and clients will be disappointed if they hand over strategic work to data-services firms if their account teams "don't have access to the right resources to get that done," she said.

"I don't feel threatened" by data firms building out agency services, said Todd Cullen, global chief data officer at Ogilvy & Mather. "I don't see their ability or intent to use the data for anything besides direct-marketing campaigns," he continued.

But Barb Kittridge, CMO of data and digital marketing services firm Cardinal Path, thinks agencies themselves have created an opening for data companies. It's "in part a response to clients' frustration by what their agency partners have been able to deliver," said Ms. Kittridge, whose company sometimes partners with Acxiom. Because data firms traditionally have access to their clients' CRM and other proprietary data through their management services, she suggested they could win business away from agencies that sometimes have trouble getting their hands on that information.