Yesterday I went through a traffic analysis of my site, and saw 74 referrals from stumbleupon. I haven't gotten to the social bookmarking aspect of marketing my site yet, so I am unfamiliar with the way stumbleupon works. Would the 74 instances of stumbleupon hitting my site be because of a crawl?
no they will not crawl your site. they are not Search Engines. Some of the stumbleupon users may reviewed / graded your site. That's why. most of the stumbleupon traffic are useless. they will stay for few seconds and will leave.
I would think those are referrals, actual "hits" from people stumbling. When a person "stumbles" they are served a random site based on the topics and interests they gave when they sign up. The system serves sites and the user gives the site a thumbs up or down based on if they liked the site or not. The system learns the users likes and dislikes and becomes better at predicting what the user may like to see. Hope that makes sense. I do not think Stumbleupon crawls, that isn't what its purpose is.