The Kenexa suite of products offers a "perfect hand-and-glove fit" with IBM's products and services "with almost no overlap," he said.

Kenexa is the latest company in the enterprise HR software market to be acquired recently. Oracle bought Taleo in February, while SAP snapped up SuccessFactors in December.

Paul Hamerman, a Forrester Research analyst, said that, after reading the deal's press announcement and participating in an analyst call with the companies, it seems to him that IBM is mostly interested in Kenexa's IT consulting services and in integrating Kenexa's applications with IBM's enterprise social networking software.

In that sense, IBM got what it wanted -- a company with a substantial IT consulting business and applications IBM can integrate with its social software, he said. "The recruiting business has changed and has heavily embraced social networks as a vehicle," Hamerman said.

Lisa Rowan, an IDC analyst, said this market has experienced strong growth, and larger vendors like SAP, Oracle and now IBM are buying their way into it.