CareerBuilder's Monk$-$e$-$mail site allows users to send personalized, private themed e$-$cards for all occasions, and users have sent more than $100$ million Monk$-$e$-$mails. CareerBuilder is using $\_\_\_\_\_\_\_\_$ to promote its services.

Question $5$ options:

viral marketing

opt$-$out marketing

customerization

niche marketing

buzz marketing