Word of mouth always works far better as you're always going to look at something sent to you by a friend or something that a celebrity you like is involved with.
One thing leads to another and it goes viral. If ten people download that app on one day then each day the new people who downloaded the app get ten new people to download the app. Within a week you could have had the app downloaded 1bn times!
That's word of mouth marketing it increases exponentially with the increase in users.
News 
Market Data 
Discover 
