Errr, have you read the Libertarian platform? It's not socially liberal in the least.
Platform - Oh I don't know - maybe the items listed under 1.0 Personal Liberty and possibly some of the items under 3.0 Securing Liberty. Despite the attempts of the left and Hillary shills to one-up Orwellian language, only one party has consistently supported your right to be free to do what you will with your body, associate with, or marry, whom you choose, truly engage in free speech, etc.. One could also make the case that their fiscal responsibility is socially liberal in that it gives you the
freedom to support what you choose, rather than having it mandated.
Government control of social services and welfare does not equal 'socially liberal'