Social networks may replace emailing in some instances, but there is room for both forms of communication.
In fact, they do a pretty good job of complimenting each other.
When to use one or the other is a question of purpose.
Social networking has become more popular in personal communications, probably thanks to the technology advances.
Nowadays, we are always connected, whether we are sitting at a desk or on a mobile phone.
The always-on connection, has created a host of new ways to communicate that are much faster than email, and more fun.
In the other hand, email remains the choice by excellence in professional communications.
One of the reasons for it is that email lets us send a well-planned message.
In marketing purpose, the social networks provide new ways of outreach yet it lack of the universal capacity email has.
The reason for it to happen is that email reaches more people, because everybody on the internet have an email account -nearly all sites on the web that require registration require an email address- yet lots of people don’t use social networks at all.
Personally, I believe social network has gained ground in communications but has not replaced email as a communication channel.