How many friends can one person really have? Facebook's resident sociologist, Cameron Marlow, found that the average male user of the site leaves comments on only seven friends' pages, the average female user on 10. But the upper limit for a cohesive social group is much higher: Oxford anthropologist Robin Dunbar famously hypothesized that the number of humans in a stable social network tops out at around 150. (Even that so-called Dunbar number is regarded as conservative.) In any case, expanding your social circle beyond a tightly knit clan is precisely where Facebook shines.
Think of it this way: There are essentially two different classes of pals. 
In sociological lingo, bonds among old school friends and family are called strong ties; these are people you keep up with on a regular basis and lean on in times of trouble. Networks like Facebook, LinkedIn, and MySpace are actually designed to exploit the second class of bonds, known as weak ties—people you kind of know but wouldn't chat with. When it comes to these looser connections, the experts' advice is, basically, go wild. Stanford sociologist Mark Granovetter found that weak ties are more likely than strong ties to provide new information and opportunities—like a job—because the resources of your 150 closest pals are probably very similar to your own.
Just watch out for nonexistent ties. Cluttering up your Facebook feed with quizzes from virtual strangers is about as intimate as taking the SAT.
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