Yes.. I would think people does not appreciate ad sent to them through "friends".. however, you can buy ad from the network, myspace, facebook, and soon twitter will let you sell to their users in general.. that would be more acceptable practice
If worded properly social media marketing doesn't have to sound like advertisements...you should definitely try to offer some valuable information when posting on social networks...which should make some people want to check you out.
Its just like pay per clicks right? But I think we don't have to pay there if we work intelligently with the site. Yeah. you are correct. It's all depends on the way we treat the ads to look like normal messages and interesting...