SAN FRANCISCO (AFP) - OkCupid on Monday said it messed with members' minds a bit in a bid to refine the formula for finding love at the online matchmaking service.

OkCupid co-founder Christian Rudder made a case for the practice in a blog post entitled "We Experiment on Human Beings" which came on the heels of criticism of Facebook for tinkering with posts to see how it influenced emotions of members of the leading social network.

"We noticed recently that people didn't like it when Facebook 'experimented' with their news feed," Rudder said.

"But guess what, everybody: if you use the Internet, you're the subject of hundreds of experiments at any given time, on every site. That's how websites work."

Rudder described how OkCupid hid profile text to find out how important words were compared to people's pictures.

It also told some incompatible folks they were ideal matches to determine if the power of suggestion played into romantic equations.

OkCupid learned that words in profiles mattered little next to pictures, and that telling people they were fine matches ramped up the tendency for them to agree.

"When we tell people they are a good match, they act as if they are," Rudder said.

"Even when they should be wrong for each other."

Worried that the OkCupid algorithm might be to blame for seemingly contradictory outcomes, the matchmaking service tested telling some seemingly perfect pairings they were lousy fits.

It turned out those people were inclined to defy the power of suggestion and grow enamored of one another anyway, according to statistics posted at OkCupid.

Rudder maintained that while OkCupid had been successful, there is much to figure out when it comes to matching people.

"OkCupid doesn't really know what it's doing," Rudder said. "Neither does any other website. Most ideas are bad. Even good ideas could be better. Experiments are how you sort all this out."

Among Rudder's conclusions from experiments was that people are as shallow as their technology permits.

British authorities early this month revealed plans to investigate Facebook over an experiment which manipulated the feelings of users, as the social network apologised for its poor handling of the row.

Facebook clandestinely altered emotional content of news feeds of nearly 700,000 users for one week in 2012 without their knowledge, in order to test whether it altered their moods.

News of the experiment caused outrage among Facebook users.

As the row grew, Facebook's chief operating officer Sheryl Sandberg admitted that the company had communicated badly on the experiment.

The research, published last month, involved Facebook giving some users sadder news and others happier news in order to better understand "emotional contagion".

Researchers wanted to see if the number of positive or negative words in messages the users read determined whether they then posted positive or negative content in their status updates.

It did not seek explicit consent beforehand, but claims its terms of service contract with users permits blanket "research".

Users, however, questioned the ethics of the study with some calling it "super disturbing," "evil" and "creepy".

"Facebook secretly manipulated the users of the site, attempting to alter their emotions," read a post in chat forum below the OkCupid blog post.

"All I can see here is honest curiosity and a desire to improve the site. Completely different things."