Seth Godin lists six points of things that people should do or ask themselves before referring someone or something to friends and family. It neatly explains why you don’t get as many referrals as you would like – even when people like your service. By extension it probably explains why you aren’t asking for enough referrals.
Correspondingly, you should consider the social media ‘like’ or ‘heart’ a very soft method of indicating preference. This is not as intrusive as a hard referral but it has led to the fastest adoption of services humanity has ever seen – so there might be something in that.