I've just been to a fascinating seminar on Facebook, social-networking and social capital. Amongst the many things I learned, FB users have more general and 'bridging' social capital than non-FB users. This means that they're more likely to respond to requests for information or ideas - and be responded to - than others.
So if I posted a request for technical help, it's more likely that a Friend of a Friend would respond. I'd assume that this applies to other social media too, but what they have in common is that SNS enables you to maintain a wider network of 'weak' social ties, relationships you'd only call on when you required a specific response. The more you post, comment and tag (these are Signals of Relational Investment, and perform the same role as chimpanzees grooming each other), the greater bridging social capital you have, and the more likely you are to receive responses when you need them: this is partly because Friends' Friends' see your posts on other peoples' walls, whereas posting on your own space is only seen by your Friends. (Does this make sense? I'm not on Facebook, so I'm slightly hampered).
What proportion of your Friends are actual real-life friends? According to Ellison's research, it's about 25%, though she didn't have the time to define friendship, and I suspect it's a very subjective term. I certainly have people I consider to be genuine friends, despite never having met them, while we all have 'friends' who might otherwise be called acquaintances.
There was also a section about online dating: apparently all users both lie, and expect people to lie on their profiles. Small lies are forgiven. Which makes me wonder if telling the truth on a dating site might actually seriously handicap the honest seeker of love.