Crescens and Titus (the one the letter was written to) also left Paul.   I think it is safe to assume that they pursued ministry in the locale that they returned to.   

We must always recognize the place where other believers are and the fact that they are responsible, not to us, but to their Master.   We can and most likely will be disappointed by people.   That is why it is critical that our trust is in the Lord.   

Marcus, Aristarchus, Demas, Lucas, my fellowlabourers. - Phm 1:24 These men were traveling and working with Paul to minister the Gospel.

Luke, the beloved physician, and Demas, greet you. - Col 4:14 Luke and Demas were traveling with Paul and assisting him in his ministry of the Gospel.

For Demas has forsaken me, having loved this present world, and is departed unto Thessalonica; Crescens to Galatia, Titus unto Dalmatia. - 2Ti 4:10 The world that Paul is speaking of is the world that the Lord created - the blessings of earth and the way of life that most people enjoy.      Paul did not say demas had forsaken Christ - only him.     Many Christians do not commit to the work of Christ as Paul and Timothy did.     They are more like demas who, not having the same vision as Paul, serve the Lord in a less impactful way.      We must refuse to equate service with salvation.     salvation is a gift.     service and reward are a function of relationship, calling and vision.     After all, whatever we do that passes the muster of the Holy Spirit fire is the work of the Holy Spirit and not us.