Any Youtube video that has achieved:
1. Millions of hits instantaneously.
2. A cult following or numerous parodies on other televised media.
3. Provoked numerous responses or imitations in a short amount of time.
Common examples of instant Youtube sensations include the Star Wars kid, Crazy frog, Cadbury gorilla and recent eyebrow advert, Sarah Silverman's "fucked Matt Damon" sketch, etcetera. Basically any video that matches the definition.