Earlier research has shown that there is a limit to how many friends a human can have. This is because our brain's ability to process multiple relationships creates a natural group size of 100-200 people. This is called the Social Brain Hypothesis. This group size is also constrained by the time required to maintain relationships - we only have so much time to devote to meeting or talking to people.

What he found was that even among regular social media users, the average number of friends they had on Facebook was 155 in the first survey and 183 in the second, right in the bracket predicted by the Social Brain Hypothesis.

Women had more friends than men (In the first sample, women averaged 166 and men just 145 friends; in the second, it was 196 vs 157), while - perhaps unsurprisingly - older generations had fewer friends than younger ones.

The first survey group, made up of regular social media users, considered only 28% of their Facebook friends to be 'genuine' (ie close) friends. When asked specifically how many people they would turn to for support in a crisis and how many they would turn to for sympathy, on average those groups were just 4 and 14 friends respectively, matching the offline findings of the Social Brain Hypothesis.

While a few people did have much larger groups of online 'friends' on Facebook, they had similar sized support and sympathy groups to others. This suggests that when social media seem to allow someone to have more friends, it is because looser acquaintances were being included in the 'friend' category, partly because social media sites tend not to differentiate between close and more distant relationships - even though we clearly distinguish between friends and acquaintances in the offline world.

Professor Robin Dunbar explained: "Social media certainly help to slow down the natural rate of decay in relationship quality that would set in once we cannot readily meet friends face-to-face. But no amount of social media will prevent a friend eventually becoming 'just another acquaintance' if you don't meet face-to-face from time to time. There is something paramount about face-to-face interactions that is crucial for maintaining friendships. Seeing the white of their eyes from time to time seems to be crucial to the way we maintain friendships."