This is an interesting write up which shows how a lack of standards between Internet giants can lead to the divulging of a person’s phone number providing you know their login id and try out the forgotten password flow.
You can download it from the following link: https://packetstormsecurity.com/files/download/122343/harvesting-numbers.pdf
Source: https://packetstormsecurity.com/files/122343/Harvesting-Phone-Numbers-Using-Facebook-And-Yahoo.html