For a project to survive, it needs support. Social media is the quickest and easiest way to get attention and supporters for projects by encourage the developers and its users to share news and information about it. With most of the world's population on some form of social media, its never been easier to spread the word about developments across borders promoting the ecosystems.
Social media airdrops sole purpose is to reward new and existing users by having them promote their digital currencies throughout social media to promote the network to new users.
Social media airdrops are one of the primary methods of promoting exchanges, networks and digital currencies because of their potential reach. It's not impossible for one person to reach tens of thousands.
Developers encourage users to use their networks to promote their technology. The wider the reach, the greater the rewards. It's possible to pay to promote for a wider reach but everyone has their own strategy.