“It calls into question how many friends we have and what they are,’’ says Turkle, author of “Alone Together: Why We Expect More From Technology and Less From Each Other.’’ “You can have 3,000 friends who look at your photos and what you’ve published, but only 100 who know about your heart.’’

MIT sociologist Sherry Turkle, who studies technology and its cultural impact, maintains that “friend’’ has become “contested terrain’’ linguistically as social media sites alter the term’s very DNA.

“The meanings of words derive from how we use them — and clearly as the world changes, we apply words in different ways,’’ says Lera Boroditsky, a Stanford University psychology professor and language expert. Before online interaction became a routine part of daily life, she adds, “You saw friends in person or spoke to them on the phone. Today there’s a real change in how we interact, and our language is struggling to keep up.’’

What we mean when we talk about friends and friendships these days has many of us baffled, not just Oliver. Experts who track the changing meaning of language agree that our common reference points are becoming less fixed as the lines blur between the virtual and real, the face-to-face and Facebooked. Between what may feel good to hear — or quantify, in the case of online connections — and what squares with the reality of interpersonal relationships.

Her toddler-age son faces his own challenges in determining who’s a friend and who isn’t, according to Oliver. “At his daycare center, they’ll say things like, ‘Johnny, we don’t hit a friend, do we?’ ’’ she says. “Some might say that’s nice to do. But what are kids growing up to understand? It becomes a label without deeper meaning.’’

“To me, the term is constantly in flux,’’ says Charlene DeLoach Oliver, a local blogger ( CharleneChronicles.com ) on health and fitness issues for young moms. “A teen, for instance, will have a completely different perspective on who’s a friend than my grandmother will. To her, a friend is someone she sees once a week, in person. Me? I have virtual ‘friends’ I’ve never even met.’’

What the term “friend’’ signifies seems harder to pin down, at any rate, having been Facebooked (ugh) into a transitive verb and overworked to the point where compliant preschoolers are encouraged to call everyone at school a friend (indeed) whether that label truly applies or not.

These days? If not exactly a dime a dozen, friends are not what they used to be.

Back in the mid-1990s, when online relationships were barely a blip on anyone’s computer screen, songwriter Randy Newman earned an Oscar nod for “You’ve Got a Friend in Me,’’ a jaunty, sentimental hymn to childhood fellowship. On television, the NBC sitcom “Friends’’ hit the ratings jackpot with its cast of young urbanites bound together by proximity and camaraderie.

The challenge, she contends, is to avoid confusing virtual friendship with the real deal. “Friendship is about letting something happen between two people that’s surprising and new,’’ says Turkle, whereas social networking “gives the illusion of companionship without the demands of intimacy. It’s friendship on demand, when I want it.’’

Is there a limit to the number of real friends one can have? British evolutionary anthropologist Robin Dunbar suggests there is. Based on his research into primate behavior, Dunbar posits a limit to the number of stable relationships an individual is capable of maintaining, and fixes it at around 150, a quantity widely known as Dunbar’s Number. He further maintains that on average we each possess about 50 friends, 15 “good’’ friends, and a mere 5 that can be categorized as “intimate.’’

In a recent New York Times op-ed column, Dunbar addressed the Facebook-friending phenomenon and questioned how authentic many of these Web-based friendships really are. “Put simply,’’ he wrote, “our minds are not designed to allow us to have more than a very limited number of people in our social world. The emotional and psychological investments that a close relationship requires are considerable, and the emotional capital we have available is limited.’’

Dunbar elaborated on that theme in an e-mail from his University of Oxford office. “I don’t think the real nature of friendship has changed as such,’’ he asserted. “The key contrast is between friends (and relations) and acquaintances.’’ Real friends “require time and effort to get to know,’’ he added, in contrast to less intimate relationships that by definition are less demanding to sustain and manage.

Differentiating between true friendship and the more superficial kind is not merely the province of academics, to be sure. As the meaning of “friend’’ gets stretched and changed, many admit to making conscious decisions about who fits the definition and who does not, and why.

To Boston-based public relations consultant Caron Le Brun, friendships get complicated by factors like age and professional contacts in real-world situations.

“I’m of the (over-60) generation where family and friends are your most valued possessions,’’ Le Brun says. “In the business world, though, especially public relations, you end up being a friend to everyone — but not really.’’

Like Le Brun, Boston attorney Joseph Feaster formed his ideas of friendship long before Mark Zuckerberg came along. Feaster, 61, has found it useful to divide relationships into four categories: partners, meaning people he would give or do anything for; friends, or people for whom he’d do just about anything (emphasis on almost); associates, meaning those he’s merely acquainted with, perhaps through his office or gym; and people he knows, but that’s about it.

He does use social media sites like LinkedIn and Facebook, the latter a site on which he counts 200 friends. However, Feaster is selective about whom he elects to “friend’’ and doesn’t look to social media sites to cultivate relationships he’d put in the “partner’’ category.

“It’s too impersonal for me,’’ he says.’’ I want to see, touch, feel you. I’m of the generation that would rather pick up the phone than text.’’

Boston University junior Sam Davidson has formulated categories of his own, divided between what he calls “primary’’ and “secondary’’ friends. The former, says Davidson, 22, are “people I’d hang out with outside class, whose phone numbers have.’’ The latter? “People I say hi to, or friend on Facebook, but that’s about it. The reality is, in college if you’re invited to a party, you can’t bring all your friends along. So that narrows it to two or three.’’

One expert who doesn’t seem overly troubled by how “friend’’ keeps changing is linguist and language columnist Ben Zimmer Because words are endlessly flexible, Zimmer says, we shouldn’t expect them to remain immutable but to be used in various ways for various purposes over time.

“People worry that this dilution is impacting (its meaning),’’ says Zimmer. “I see it as the inherent flexibility of language taking on new guises over time.’’

Society often focuses on these semantic shifts, he adds, as a way to complain about larger social phenomena, such as being disconnected geographically — and emotionally — from one’s family and childhood roots. “Words become proxies for anxieties,’’ he says, “in this case anxiety about connections to people’’ in the Facebook age.

Joseph P. Kahn can be reached at jkahn@globe.com.

© Copyright 2011 Globe Newspaper Company.