You get a bonus for inviting your friends and then they have to invite their friends.
"You get a number of shards for sharing on social media, but you can simply click the task, let it redirect you to the site, then navigate back and it will register as though you actually did something, even if you never actually post."
"Destiny stops being particularly "FUN" Solo. Friends or Quick acquances makes Destiny more fun. Fireteam XP Bonus only active with at least one other player not solo."