I'll tell you one way a lot of indie fashion brands, mobile apps, and bedroom music producers are doing it right now.

There are people (mostly teenagers/early 20s) who set up catchy Twitter accounts with endless streams of funny photos or videos along various themes. They cross promote and cross retweet these accounts (often retweeting each other's posts for just 5 minutes, then unretweeting again to remove the trace) until they build up over 100K legitimate followers. They then sell "spots" on these accounts to (mostly) people running Shopify fashion stores or mobile app developers ($18 per 100K impressions is a typical rate right now). I've also seen a couple of musicians "come up" through this approach using Soundcloud. (What they do is have fake female accounts repost Vines, tweets, or screenshots of Tumblr raving about thing X, then get those retweeted on "decks" - people then impulse retweet, then they delete the original content to make it all look organic.)

This is the wild west of social right now, it's hard to find info about it, you gotta spend weeks digging around and warming up to the right people, especially since a lot of the trail is destroyed shortly after it's posted.

I'm not going to go into much more detail as I've spent months figuring out this space, but believe me, there are some amazingly savvy 15-25 year olds out there making things on social not look exactly what they seem(!) :)