Word of mouth = peer to peer advertising
There are some numbers that show word of mouth is more effective than any other form of advertising. Specifically, i can say that peer to peer word of mouth is the most effective, and if the users are generating this type of content, then the other users are likely listening...
Response Mean Index*
Positive word of mouth from someone you knew personally 7.7 - 134
Negative word of mouth from someone you knew personally 7.0 - 121
A negative news story on TV/Radio/Newspaper/Magazine 5.7 - 99
A TV or radio commercial 4.7 - 82
An advertisement in a newspaper or magazine 4.6 80
Source: WOMMA