As Ingo writes, the mail problem is that you only know about people who respond to your survey, and don’t know about non respondents. So you do need to find out any info about the demographics of social media users in general, and compare your sample. In the meantime, in any reporting, carefully describe your respondents, and only write “the people who responded to this survey said … Also, you can report the responses from the people in the one region separately, and combine all others.
Can you advertise your survey in multiple ways to get more respondents? What is it about? If you advertise it through your academic affiliation, you may get more respondents, since people may see it as more legitimate.
Thanks
Gene Shackman