A lot of people don't seem to get Snapchat, so I wrote a blog post about it here: http://www.brianchu.com/blog/2013/09/11/photo-embarrassment/.
Have people actually met/seen others using Snapchat? I'm in college so tons of my friends use Snapchat regularly (about every couple of hours). Snapchat is essentially used for picture messaging of frivolous images, almost like a picture-status-update of something funny or current. Nobody I know really uses it for "dirty" pics, though unflattering pics (of yourself or others) are fairly common. It's almost a foil to Instagram - Instagram encourages you to post by making your photos permanent but beautiful; Snapchat encourages you to post by making your funny/ridiculous/unflattering photos ephemeral. See the difference? It's ephemeral vs permanent, funny/ridiculous vs beautiful.