Leahy and Rep. C.A. "Dutch" Ruppersberger of Maryland, the top Democrat on the House Intelligence Committee, said they were unaware of ZunZuneo.
"I know they said we were notified," Leahy told the AP. "We were notified in the most oblique way, that nobody could understand it. I'm going to ask two basic questions: Why weren't we specifically told about this if you're asking us for money? And secondly, whose bright idea was this anyway?"
The Republican chairman of a House oversight subcommittee said his panel would also be looking into the project.
"That is not what USAID should be doing," Rep. Jason Chaffetz, R-Utah, chairman of the House Oversight and Government Reform national security subcommittee, said. "USAID is flying the American flag and should be recognized around the globe as an honest broker of doing good. If they start participating in covert, subversive activities, the credibility of the United States is diminished."
But several other lawmakers voiced support for ZunZuneo, which is slang for a Cuban hummingbird's tweet.
Sen. Bob Menendez, D-N.J., chairman of the Senate Foreign Relations Committee, said USAID should be applauded for giving people in Cuba a less-controlled platform to talk to each other.
"The whole purpose of our democracy programs, whether it be in Cuba or other parts of the world, is in part to create a free flow of information in closed societies," Menendez said.
Some Cuban-Americans also applauded the effort.
"I don't think it was a bad thing if it was opening up people's minds. ... At least this way they were helping people communicate," said Miami construction worker Ivan Marrero, 48, who fled Cuba in 2005 by boat.
Others said they worried it would hurt the island's small movement of independent journalists and bloggers.