[android-developers] Identifying user - email, phone number, FB and gmail?

Hi,
I am developing an Android app but I wanted a way to uniquely identify
the user. However I dislike the idea of creating username and password
since I find that will be inconvenient for the user.
The alternative that I was thinking was,
1. Facebook login
2. Gmail login,
3. Email id
4. Phone number

My question is - Is It legal to display the a user's email id or phone
number to other users?
And, in general ur comments on whats the best approach.
My app does require that users be able to contact each other somehow.

Thanks in advance. I would appreciate you any insights you can give.
-Vishal

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