There is a great concern about the potential for people to leak private information on OSNs, but few quantitative studies on this. This research explores the activity of sharing mobile numbers on OSNs, via public profiles and posts. We attempt to understand the characteristics and risks of mobile numbers sharing behaviour on OSNs and focus on Indian mobile numbers. We collected 76,347 unique mobile numbers posted by 85905 users on Twitter and Facebook and analysed 2997 numbers, prefixed with +91. We observed, most users shared their own mobile numbers to spread urgent information; and to market products and escort business. Fewer female users shared mobile numbers on OSNs. Users utilized other OSN platforms and third party applications like Twitterfeed, to post mobile numbers on multiple OSNs. In contrast to the user's perception of numbers spreading quickly on OSN, we observed that except for emergency, most numbers did not diffuse deep. To assess risks associated with mobile numbers exposed on OSNs, we used numbers to gain sensitive information about their owners (e.g. name, Voter ID) by collating publicly available data from OSNs, Truecaller, OCEAN. On using the numbers on WhatApp, we obtained a myriad of sensitive details (relationship status, BBM pins) of the number owner. We communicated the observed risks to the owners by calling. Few users were surprised to know about the online presence of their number, while a few others intentionally posted it online for business purposes. We observed, 38.3% of users who were unaware of the online presence of their number have posted their number themselves on the social network. With these observations, we highlight that there is a need to monitor leakage of mobile numbers via profile and public posts. To the best of our knowledge, this is the first exploratory study to critically investigate the exposure of Indian mobile numbers on OSNs.