Requesting to follow someone on social media in order to use them as a source is ethical, in my opinion, as long as you are making it known that you are a journalist and the reason was to strictly ask them questions for your story. The relationship between the source should be professional so that there is no bias towards your reporting.
An example of journalists using Facebook for sources would be Tracy Swartz, a writer for The RedEye in Chicago. She writes about the transit system, and in this case, bus drivers friended her so that they could send her information. In this case, however, Facebook was a valuable tool in order for her to find out more information.
In this day and age, social media is important for promoting stories, reporting updates, as well as getting information from followers. Someone from social media can send a reporter a video of a fire that happened, for example. The reporter can ask that person questions to find out more information.
NPR has a code of ethics for journalists when it comes to social media. In it, they say that social media can be used as a way to speed up research and news gathering, as well as extend a reporters contacts.
However, in it they say reporters must carefully cite exactly where and who they got the information from, and make it known if they were unable to confirm something. They must also verify pictures from social media, as it is easy to manipulate images. If they are unsure, they should not post it. They should also follow up to confirm information they got from an online source by calling them on the phone or by doing an in person interview.
When getting information from social media, there is also the chance of appearing less credible . There was a study done by the Amsterdam School of Communication that found the public viewed sources obtained from phone calls and in person as more credible than from Twitter or Facebook.