How should exoplanet discoveries affect SETI? Given that SETI was created before the discovery of exoplanets, has this changed how they search? Would it make more sense to focus efforts on discovered earth like exoplanets?
Well you are correct and of course that kind of research has been going on. The SETI programme targeted a set of Kepler stars with evidence for a planet (or planet candidate) in the "habitable zone". The initial searches for narrow-band radio emission (at 1.1-1.9 Hz) appears to have concentrated on those systems containing multiple planets.

The (null) results for $\sim 100$ system were published by Siemion et al. (2013). The observations were not sensitive enough to detect the sort of "radio chatter" emitted from our planet, but might have detected some sort of deliberate signalling beacon.

A slightly more imaginative approach, which I'm struggling to find the details of, is that one can observe Kepler multiple planet systems when one calculates that two planets are in conjunction as seen from the Earth. There is then the chance of seeing communications from one planet to the other. (e.g. http://adsabs.harvard.edu/abs/2014ebi..conf..5.2K ).

In a more general sense, the exoplanetery discoveries have provided great impetus to the SETI programme. It is now clear that planetary systems are common and present around most stars. It is also now apparent that small planets are common. New possibilities are also being considered. Most exoplanets in habitable zones are orbiting stars smaller and less luminous than the Sun. No exact Earth analogues have yet been found. This has led to an explosion of activity investigating planetary atmospheres and habitability around K- and M-dwarf stars.