"What interests the megavendors are those marketing companies that have a base of large clients," he said.

Marketo has made more success with small to mid-sized companies than with larger entities.

Stephens analyst Carter Malloy said that both Marketo and Responsys could be targets. He doesn't follow privately held Silverpop.

"It's the value of the technology and scarcity of assets of this type of quality," he said.

Malloy notes that Responsys, trading at about 3 times revenues, scored higher in a recent Gartner ( IT) ranking than ExactTarget, Eloqua, Marketo or Neolane, which were acquired at or trade at 6 to 9 times sales. Marketo is at the higher range of the scale.

Before Salesforce acquired ExactTarget, some industry watchers speculated that Marketo might be the target. Responsys CEO Dan Springer suggested at a Stephens investor conference in July that Salesforce.com could have been deterred by Merkato's price expectations.

"Many people thought, and I definitely considered this, that they would buy Marketo, who was the next largest B2B centric player in the space who has recently went public," Springer said.

Salesforce may have felt squeezed after its $689 million purchase of Buddy Media in 2012.

"They paid so much money when they purchased Buddy Media for a relatively small amount of revenue, to go out and try to get what Marketo was trying to get in the marketplace was going to be very difficult," Springer said.

"For them to spend that much money again for relatively small revenue," he added, "risking that they wake up a year from now, and in the investment community, you all would say 'You guys are writing huge checks and not building very big businesses.' "

Responsys, like Neolane, has 400 customers. The San Bruno, Calif.-based company generates close to $200 million in revenue from retailers such as Sears Holdings ( SHLD) and Lands' End; airlines Southwest Airlines, Deutsche Lufthansa and United Continental Holdings ( UAL) and financial services providers E-Trade Financial ( UAL) and Allstate ( ALL).

Atlanta-based Silverpop raised $25 million in April, but did not disclose terms of the deal. It has backing from NeoCarta Ventures, TTV Capital, Draper Fisher Jurvetson and D.E. Shaw. It leans more heavily toward business-to-consumer than business-to-business marketing, Brown said, through mass e-mailings.

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