Eyelashes are some of the more delicate hairs on the body, and they can easily be caused to fall out prematurely if they are manipulated too excessively. For this reason, by far the most common cause of eyelashes falling out is the excessive application of eye makeup and mascara, as well as its removal. If you use these products, you may want to stop for a while to see if that makes a difference in terms of your eyelash thickness.
Another common condition that can lead to eyelashes falling out is blepharitis, which is an inflammation of the eyelid edge. Again, blepharitis is most commonly associated with makeup use, but really any chemical irritation may lead to blepharitis, which is characterized by redness and puffiness right at the eyelid edge. Once again, however, treatment focuses on discontinuing possible offending agents (such as eye makeup) and warm compresses to the eyelids to reduce inflammation.
If you are losing hair all over your body (such as on your scalp) and not just eyelids, this may be a sign of a more systemic medical problem. Either way, I recommend that you speak with your primary care doctor
or a dermatologist
to rule out more serious concerns.