1) There is a section for Photo Biz here, please read existing threads (and there are a few) who address this topic.
2) The answer depends on the legal context of the country, an answer valid for Singapore will not be automatically valid in other countries. Do check this thoroughly.
3) A model release is not commonly required and used in Singapore. Whether the model can use such a picture on a social website (or any other website) depends on the terms and conditions set out in the agreement between photographer and model.
You're asking as a model or a photogtapher?
If you are a model, how do you get hold of the photos? And since you have the photos, I see no wrong of sharing in social media sites.
If you are a photographer, you should make clear the usage before you pass any images to your model.