StumbleUpon calls itself a discovery enginebecause, at one time, $50\mathrm{.}27\%$ of all traffic from a top $10$ list of social sites came from StumbleUponTheir relatively small group of users $(15$ million$)$ often out$-$refers the larger user base from Facebook $($over $1\mathrm{.}15$ billion$).$ In fact, the influential users of StumbleUpon generate $1$ billion page referrals per monthTheir users, called Stumblersintroduce other Stumblers and non$-$users to web content, providing a map to an adventure you wouldn't otherwise have found out aboutThe influence of these Stumblers was deemed valuable by film marketerswho worked with StumbleUpon to promote films like Jurassic Park $3$D and Evil Dead. Stumblers are young and highly engaged, so studios see the personalized search engine, which learns the preferences of its Stumblers, as a way to reach early influencersIn this case, the influencers are expert informal sources of information, also known asA. $.$Salespeople b$.$ opinion leaders c$.$ Buzz agents d$.$ Spokespeople e$.$ consumer Reports