Eye brow loss is due to many reasons, such as rubbing eyebrows from habit, or vigorously applying eyebrow pencil can cause eyebrows to fall out or sometimes aging may be another reason. I had a same issue, my eye brows started thinning and then it fall out. Thus I checked with CHTC (www.chtc.ca) and they told that it is Telogen effluvium. It is a temporary condition where excessive hair falls out; it can affect eyebrows as well as scalp hair. I am under their treatment for last two months. They suggested me to use Eyebrow Rogaine and that may be helpful in diminishing hair loss.