By joining the registers, Facebook users have helped to widen the pool of bone marrow and blood donors available and increase the chance of a match being found for patients.
The parents of 23-month-old Iona Stratton, who fell ill with leukaemia when she was 13 weeks old, launched a Facebook appeal in October to find a bone marrow donor.
The appeal attracted almost 7,000 members and they found a match in Australia within weeks.
Iona died this month due to complications after her transplant, but the case highlights how the social networking site is being used to help raise awareness.
Finding a donor for Iona was especially difficult because of her mixed Chinese and English heritage. There is a lack of people from ethnic minorities on registers worldwide according to the Anthony Nolan Trust leukaemia charity.
Claire Stratton, Iona's aunt, said that due to donor anonymity it was hard to tell whether a donor had been found directly because of the Facebook appeal, but the appeal had led to people joining registers or giving blood at private clinics.
Facebook appeals are being launched to help other children in Britain including 15-month-old Bethany Dawson who is waiting for a liver and bowel transplant. More than 155,000 people have joined her group, which is called Help Save My Daughter Bethany and encourages members to join the NHS organ donor register.