Do you want to NASCAR-up your login page? Do you want to have your users’ login credentials stored in a third-party service? Do you want your brand closely associated with other brands, over which you have no control? Do you want to add additional confusion about login methods on your app?
Sometimes it makes a lot of sense, and other times it’s just not worth the trade-offs. But don’t use them because they’re on every other popular app. Use them because they serve a purpose for your business and your users.
I find myself logging in via FB and Twitter less and less upon downloading apps I think I’ll use in the long-term. I prefer to establish a one to one relationship.
FB/TW log-in is convenient but messy. As a user, social login really depends how long I’ll think I’ll use that particular app or page.