Most of the folks on my Facebook feed have a picture of their kid, somewhere they've gone, a comedian, etc. Probably 40% have a reasonably good quality image of themselves.
Hackathons, yes. The selfie aspect makes it a fun/ceremonial part of the experience of the event. 1 day before, you get the email, you take your pic, and you get excited for the event the next day.
It would be interesting to have a post-event-page that has a gallery of all the selfie tickets after the event so that attendees could go there to see all who attended if they wanted a reminder of who they met and who to reach out to. Use my selfie ticket as proof i attended and as a request for the contact info of another attendee; "I was at the event as well, and wanted to get your contact info; here's my selfie ticket as proof I was there!"