You already know the stuff, mostly it's the terms are different.
Mostly, it's all about who you know, and who they know, and talking about stuff.
(Facebook calls the who you know/who they know part the "social graph". They call the stuff that you might talk about, a "social object.")
If you're doing it online, that just means you're connected and talking to more people more often than in everyday life.
Also, you're connected to way more friends of friends.
Also, what you say is transmitted instantly.
The downside is that disinformation travels fast and sticks. Watch out for astroturfers.
… and yes, I've oversimplified, but this is the gist, in the plainest language possible.