For only the tiniest benefit to me (I guess it's marginally faster to sign up, maybe?) and no real benefit to you, your service is now beholden to another. What if Twitter bans a user that is valuable member of your site? What if Twitter goes under? What if Twitter is outlawed in some countries? What if I have a moral obligation to signing up for Twitter in the first place?
But one of the big advantages is the one click sign up, no need to add profile picture etc.
Plus email sign up takes too much time and filling information that people might not like to fill.
Overall I agree. But what alternative do we have?
With the twitter flow, I have to click to sign in, then read what permissions you're requesting (from another comment I hear that you aren't requesting permission to tweet for me, which is GOOD), then click to accept.
Since I personally don't require email, the user actually has to give me zero personal information. Using twitter, on the other hand, I have to give you my twitter handle, which might actually be personal information. I don't know how you're going to USE that information, so if it's personal, I'm not sure if that information is going to be shown to everyone in the community or not.
What do you mean in your site the email is OPTIONAL? How can user reset their password if they forget their password? IF a user sign up using their email then they have to confirm it as well which makes the user to log in to his email.
Also users have one less password to remember if they use Facebook or Twitter sign in?
What if the site needs a profile picture for user profile? Then the signup process takes longer time and users have to fill a larger form.
I'm not saying I'm totally in for Social signups, but what alternative do we have?
Let's be real. Most users reuse their password for everything anyway. Those that don't likely use a password manager of some sort.
I doubt that any site needs a profile picture. Even Twitter gives you a default if you don't want to set it.
The alternative we have is what we did before social was a thing. The Internet existed before Twitter. The very site we're on doesn't require a social pairing to sign up for it!