As a small brand or retailer, it’s tougher than ever to stand out on social media. Consumers, weary of the advertising deluge on social networks, are getting choosier about whom they follow on social channels. And mega-retailers boasting huge marketing budgets post original content numerous times a day, effectively gobbling up major swaths of the so-called attention economy.  But many experts say there’s a way to level the playing field: influencer marketing, the practice of pairing brands with social media celebrities to create alluring, organic-feeling content. The concept (which is really a modern riff on celebrity endorsements) is simple. But it’s become a key strategy for many trendsetting brands, including Birchbox and Madewell, that regularly team up with high-profile social stars for brief, targeted campaigns.   Influencer marketing’s high return on