Ninety percent of people identified in a tranche of communications intercepted by the NSA were ordinary internet users, not surveillance targets, analysis by a US paper suggests.
The Washington Post says innocents were "caught in a net the National Security Agency had cast for somebody else".
Much of the highly personal information was retained, the paper says, even though it had no intelligence value.
The information was provided by former NSA contractor Edward Snowden.
The paper said it reviewed some 160,000 emails and instant-messages and 7,900 documents from some 11,000 online accounts, gathered by the NSA between 2009 and 2012.
The Post said that a four-month investigation it carried out revealed that nine out of 10 of the account holders - including many Americans - were not the intended surveillance targets.
Much of the information has, the paper says, a "startlingly intimate, even voyeuristic quality" telling stories of "love and heartbreak, illicit sexual liaisons, mental-health crises, political and religious conversions, financial anxieties and disappointed hopes".
However the paper says that the intercepted files also contained "discoveries of considerable intelligence value".
These included "fresh revelations about a secret overseas nuclear project, double-dealing by an ostensible ally, a military calamity that befell an unfriendly power, and the identities of aggressive intruders into US computer networks", it said.
The Post argues that the surveillance files highlight a policy dilemma for President Obama - while there are some discoveries of "considerable intelligence value" there is also "collateral harm to privacy on a scale that the administration has not been willing to address".
Snowden, 30, fled the US in May 2013 and has been living under temporary asylum in Russia.
Last year, he fed a trove of secret NSA documents to news outlets including the Washington Post and the Guardian.
Among other things, the leaks detailed the NSA's practice of harvesting data on millions of telephone calls made in the US and around the world, and revealed the agency had snooped on foreign leaders.