Jay Leno is known for opening every “Tonight Show” with a monologue that consist of a series of one liners about current news events. Upon taking over Jimmy Fallon should abandon the practice writes Esther Breger:

• When the monologue first started it was a fun and useful way to contextualize the news and make it humourous.
• However in the age of Facebook and Twitter people read funny one liners about events as they’re happening. By the time the Tonight Show starts jokes about news events seem old.
• Instead what gets people fired up are viral videos and bits that spread like wildfire across video sharing sites and social news feeds.
• Jay Leno was trained in stand-up comedy. Jimmy Fallon is trained in improv comedy and he should stamp his signature on the show by playing to his strengths.

See examples of the differences in style sand read more over here.

Source: New Republic