In the sea of food pictures your friends post on Instagram, how many McDonald’s Big Macs have you seen? How many Starbucks cups, Sheraton Hotel lobbies or Budweiser beers?

Perhaps a few. But, in general, mainstream brands are dramatically under-represented in social media—a fact that illustrates a big challenge facing mass consumer brands.

People use consumer brands to express themselves and tell stories about who they are and what they are proud of. This self-expression via consumption is more true today than ever before thanks to social media, which thrives off people sharing what they’re doing, eating and wearing.

But big brands simply aren’t as suited to this new reality, and as a result, they’re increasingly vulnerable to competition from smaller companies. These smaller brands are more efficient at helping consumers define themselves for a few key reasons.