Whether you have 5,000 friends on Facebook or 50, chances are at least some of them are, well, not really your friends.

And we don't mean this in the sense that they're bots.

A newly published University of Oxford study suggests the average Facebook user may have just 14 "close" friends amongst all their connections on the social network, and only four they can actually depend on in times of crisis.

The size of the latter group, referred to by researchers as one's "social clique," was found to be consistent across all age groups between 18 and 65, in both men and women, regardless of how many friends they could boast online.

What this suggests, according to the study's author, is "heavy users of online social media do not have larger offline social networks than casual users."

Evolutionary anthropologist Robin Dunbar studies "the behavioural, cognitive and neuroendocrinological mechanisms that underpin social bonding in primates (in general) and humans (in particular)."

'Heavy users of online social media do not have larger offline social networks than casual users, even though more of these may appear online for heavy users.' - Robin Dunar, University of Oxford

His recently published work on social networking sites in the context of human relationships involved surveying two large, separate groups of adults across the U.K.

The first group had 2,000 men and women between ages 18 and 65 who "made regular use of social media," while the second included 1,375 "professional adults who worked full time" at weekday jobs, and were not necessarily social media users.

Both groups were asked a series of questions about their online and offline behaviour, as well as the size of their social networks in both spaces. The number of friends listed on the Facebook profiles of these subjects was also used as a test metric.

"These data constitute the first attempt to determine the natural limit on network size using unbiased, randomized, stratified sampling of a national population," wrote Dunbar in his findings, published this month by the Royal Society of Open Science.

"As such, this study is the first real attempt to test whether online social media do allow us to increase the size of our social networks."

What Dunbar found was consistent with previous studies into the size of typical individuals' relationship networks – five very close individuals, 15 close individuals, and additional layers of friends and acquaintances of up to 1,500 individuals.

Those surveyed by Dunbar were found to have a mean of 4.1 people in their "social clique" and 13.6 close friends.

Interestingly, however, subjects with more Facebook friends (young people and women were found to have larger online networks) did not report having more friends in general.

"Since respondents were not asked to specify the quality of their relationships with individual [online friends], we cannot say whether the extra members in these larger networks really are additional high-quality relationships or simply individuals that would normally be included in the circle of acquaintances," Dunbar writes.

"In most online platforms, these are all formally subsumed under the single category 'friends'," he continues. "Whereas in the offline world we would naturally distinguish between friends and acquaintances of different emotional quality."

In a previous research paper, Dunbar suggested humans can only ever maintain up to 150 meaningful relationships at a time, either online or in real life.

"The way in which our social world is constructed is part and parcel of our biological inheritance... We fit in a pattern. There are social circles beyond it and layers within – but there is a natural grouping of 150," he told the Guardian in 2010.

"This is the number of people you can have a relationship with involving trust and obligation – there's some personal history, not just names and faces."

So there you have it. Deleting elementary school classmates from your Facebook feed on their birthdays isn't mean – it's biology.