Here's some advice I would offer:
1) Like you, I was really nervous for my skype interview. The skype aspect takes a little time to adjust to and doesn't feel as natural as being in-person does. I would recommend that you practice having a conversation through that medium with a friend beforehand and have that friend give you feedback. For my interview, I remember feeling simultaneously self-conscious because I could see a video of myself on the screen, and uncertain whether I should be looking at a video of my interviewer's face or the camera hole to make 'eye contact.' Get used to it ahead of time so you can relax and focus your energy on the interview.
2) Your application should have already answered the big questions like "Why Sociology?" or "Why this department?" It probably did if you are being interviewed. In my opinion, I think interviews are used either to answer questions that were raised in the application, or simply to get a feel for the person behind the application. Review your application and be ready to talk more in-depth about what is there, as there's only so much space in the application to talk about it at length. Be ready to address weaknesses, but don't assume that's the only reason they want to speak with you. I have a low GRE Math score and was prepared to talk about that, but they never even mentioned it.
3) I was perhaps overly prepared to answer the usual questions (why soc, why this department, etc.) that when I got asked "If you weren't doing a PhD in Sociology, what would you be doing instead?" I was stumped. So prepare for that possibility as well!
4) Remember that at the end of the day you are choosing them as much as they are choosing you. This is an opportunity to learn from them what you couldn't gather from their website. Ask questions with enthusiasm and genuine curiosity.
5) At the same time, present yourself as an asset to the department, describe how you will complement the working being done there. Everyone has weaknesses. If you got an interview, they already believe that you show potential. Build upon that.
Good luck!