According to John Bines, director of fundraising and communications at Sightsavers, one of his employees received an email from the US charity's chief executive through the social networking website Facebook asking for the employee's 'friendship'.
Bines declined to reveal the name of the charity, but said that it had a "sizeable overlapping mission".
He told Third Sector: "I don't deny we work in a competitive environment, but people don't expect to see charities competing in that way. It shows us both in a bad light with the supporters. I think it is damaging to go to someone so directly."
However, Bines said that the offending charity had recently admitted what it had been doing, apologised and agreed to stop.
"Charities must be aware that anyone signed up to Facebook can view details of their supporters and, as in this instance, can contact them directly," said Gill Wooton, director of fundraising consultancy Wooton George Consulting.
"Sadly, there are a small number of charities and people who would continue to use information for their personal gain, and even if it were regulated, it would be very difficult to prevent overseas charities exploiting the information."
Charities use websites such as Facebook to build contacts and supporters. Anyone with a Facebook account can search for other registered individuals or organisations they are interested in and send friendship requests.
The Institute of Fundraising and Facebook's administrative team both declined to comment.
Sightsavers' discovery comes after a study by the Press Complaints Commission, the results of which were published last week, which showed that nine out of 10 people think social networking websites would benefit from tougher privacy regulations.
- Sightsavers International discovered that a US charity rival had written to its Facebook friends asking for support
- A recent Press Complaints Commission survey found nine out of 10 people think social networking sites need tougher privacy regulations.