One way to know that something is a total interweb fad/meme is that you encounter it zero times on wednesday, one time thursday morning and think, “My, that’s cute, but I have other things to focus on,” and enough times by thursday evening to decide that you’re not copying one innovator’s idea and rather just putting in your two cents. Finally, by Friday morning you have seen it in so many places that you merely point and laugh, both at yourself and others.
Post Script - You will know that the fad is over and you must abolish all reference that you ever participated in it when your “omg-i’m-so-interwebs-cool” friend randomly publicizes the concept like it’s brand spankin’ new, about a month from now. The link he/she includes in said email will link not point to the meme’s originating website/video/photo, ever.
UPDATE: I neglected an important aspect of random web fads - the inevitable, over-reactive backlash. It happened in record time on this one: less than 48 hours, I believe.