As they did in their previous polls the networks used heavily weighted Democrat polls to show Obama winning the debate. ExampleCNN:

“The CNN/Opinion Research Corporation poll was conducted by telephone with 675 adult Americans who watched the debate. All interviews were taken after the end of the debate. The audience for this debate was 38 percent Democratic and 31 percent Republican — very close to the partisan breakdown among all Americans nationwide. The survey’s sampling error is plus or minus 4 percentage points.”

Note that CNN, along with the other networks have been telling us for four years now how “evenly divided” the country was. Hmmm, must have changed since the debates started.