Social promotions are largely impacted by exclusivity and time.

Exclusivity: Offer special promotions only to those who like/follow your business, have subscribed to your email list, or have previously made a purchase.

Time: Offer promotions for only a short amount of time, such as a Flash Sale. This psychologically makes users want services now out of fear they will miss out on a good deal.

Types of promotions include: percentage off order, promo code, BOGO, referral code, upsell, added value offer, contests/sweepstakes, etc.

If you want to use this tactic to raise awareness, build social sharing into the conditions.