From what I've seen around the forum, quite a few people don't really use Facebook. I don't use it to post anything, only to keep in contact with my sister and to keep track of events. I know it's a good basis for social media research and autism, but there may be fewer participants than you'd like. As
@Judge said, places like this forum are a better measure as we seek out places where we don't feel like outsiders - and most of our lives, we've always felt like we're on the outside looking in. Because of this factor, places like Facebook seem trivial and usually don't fit with our lives to the same degree as it does for neurotypical people. A lot of us have social anxiety too, which is a contributing factor to aversion to mainstream social media.
Personally Facebook only depresses me, it only reminds me how different I am. I'm more prone to use Instagram to post art I make, as it's less interactive and there seems to be less eyes/judgement there.
I hope you take some of these factors in and reflect on ASD people's frequent comorbids, such as ADHD, depression, social anxiety and OCD which might skew your results.
I'll forward your questionnaire to my sister, she uses Facebook much more and is strongly suspected to be on the spectrum, that's the least I can do.
Again, I wish you all the best with your study.