They're earned from conversations.
Every time you choose a conversation option during a cutscene while in a party, you get Social Points. In multi-person conversations, each character will roll to see who gets the definitive answer and speaks aloud.
If your character wins the roll and speaks, you get 2 Social Points per person in the conversation (so 8 if you're in a full party). Everyone who didn't win the roll gets 1 Social Point per participant (4 in a full party).
I'm not sure if the rewards are greater in a Flashpoint, though.