Social media is a form of publishing personal information. Not just sharing, publishing. If your participants choose to publish information for the world to see, it can be used to locate them.
I once read about a bondsman looking for a client who violated parole. He had a warrant for her arrest. He found her Facebook page and saw that she planned to attend a picnic at a certain park on Saturday afternoon. I think you can probably guess the ending to this story.
Today it is even easier to find people because people publish many aspects of their lives (too much if you ask me), completely free of charge. If it will help your project reduce research attrition, go for it.