Does anyone know how the "coins" build up

How do they accumulate? and what else can you do with them? I'm not big on the whole gift thing. I'd like to be able to give them away, or use them to buy other features. Like if I get a million, I can kick a user off SW..actually, scratch that idea before people start pooling their coins. But really, what gives you the coins? if I give someone BA, will they get coins?