Social liberalism is based upon the notion of tolerance. That is, there are liberal Christians who do not believe homosexuality is as valid as heterosexuality, but nonetheless advocate same-sex marriage. It is not the state's right to tell two people they cannot be married. Let the Churches choose who they want to marry. It is a negative liberty argument.
Social progressivism is positive. It's foundation is that traditional values must be eradicated and replaced with secular humanism. Tolerance is traded for equality. For instance, social progressives advocate using the education system to actively promote same-sex equality, much in the same way education actively promotes racial equality. The perfect way to tell the difference is that social liberalism would see drugs decriminalised, but social progressives would not necessarily come to that conclusion. Positive liberties must be taken into account, not just negative liberties.