They initially recruited some really good (especially for social networks) "experts" who made the content a cut well above your run of the mill social Q&A site.

Early adopters were intrigued by this early high-quality community. "Less early" adopters were in turn attracted by the (still) high-quality content; this group was followed by people like me, who have seen the site's quality gradually decline. Heaven help the people who have followed me onto Quora, much less the people who will follow them... :)