Actually its not a question of whether mobile users in developing countries (like mine: India) can sign up for an email address or not?
Question is, whether they find it useful enough to go one/two steps extra (major portion of Internet is still in English, which is a major hindrance to majority of public in developing countries) to get an email, which they suppose to never use?
Actually mobile growth happened in these countries only when to use an app, email requirement was dispensed with.
There are full, very populous states in India, which even don’t know what an email is, and what’s its purpose, why is it necessary, and yet they use Whatsapp the whole day, every minute of the day. [I know its very very difficult to digest by the western people, but this is the truth]
So, the point is, there will be crores of people, who will join if they can join only with their mobile number. But will skip, if we mention to them that these is such thing as ‘email’ which is required to be on on certain platform (‘platform’ word, even in their native language’ will also scare them away).
Edit1: I get many users response as that they’ll meet me sometime to get our forum configured on their mobile. I ask why? Its very simple. They just have to click on the facebook or google account. But they say, FB and Gmail is always working on their mobiles, but they don’t remember the PWs of the same. And when our forum requires the PW for at least 1 time.