Yes, the Old Testament prophets were social activists. They called their fellow countrymen to return to the Covenant laws to which they (Israelites) had originally agreed. The only difference between New Testament believers and Old Testament prophets is: Old Testament prophets were part of the God-ordained Theocracy of Israel, whereas we (New Testament Believers) are not; and so, we cannot claim a legitimate Theonomy or Theocracy. Thank the Lord for the Reformation and resultant Enlightenment!
However, as the believing Body of Christ, we can and must prepare for the coming Kingdom by living like we are in it now (Matthew 5-7). As a priority, we should present the Gospel. But, we should do it in the relational contexts of agape love and grace and forgiveness. That is, we must exercise Mercy in Private and Public Life.
To expound on the theocratic, social activism of the Old Testament prophets, which (along with Christ and the Apostles) sets the example for our individual influence as believers (Matt. 5:16), please see: A Biblical Theology of Micah.