There are many social networking websites that provide online networking opportunities; all of these platforms require creating an account, then logging in and updating information to maintain a profile. Our research investigates mobile number based log in feature of both Facebook and Twitters - two exemplary social networking sites – for degree of seamlessness and other security features. Our research reveals that there are major dissimilarities as well as flaws in the deployment of mobile number based login on these two platforms. It has been observed that Twitter allows creation of multiple accounts using one single mobile number, however, logging out from one such account automatically logs out all other accounts associated with the same number. On the contrary, Facebook allows creation of multiple accounts using one single mobile number without deactivation of the previously created ones, however, if same password is used for more than one accounts associated with the same number, only one account can be accessed at any given time. This paper also provides probable solutions to address the security loopholes as identified by our research.