Researchers assessed the website and social media activity of the biggest 300 charities by voluntary income, excluding schools and grant-making trusts.
The report, Charity Web Check, scored charities according to how easy their websites were to navigate and access, and how they performed on Twitter, Facebook and YouTube.
The report says Age UK came out top overall "in large part thanks to its website, the highest scoring of all the sites examined".
It also commends the charity’s social media. "Contrary to what some may assume, Age UK has a strong social media following, with more than 12,000 likes for its Facebook page and in excess of 6,000 Twitter followers," the report says.
British Red Cross is ranked second and praised for its blog. The British Heart Foundation is ranked eighth overall but first for social media, thanks to 750,000 views of its YouTube videos and a "chatty and informal" Twitter feed with nearly 13,000 followers.
Third Sector Research is a division of Third Sector. The research was carried out in July and August.