I don't know about selection bias, but their coverage of the topics they select is extremely even handed, possibly charitable to the right. Compare these two which I just read and both of which came about within a few days of each other:
http://www.politifact.com/truth-o-m...-sanders-repeats-flawed-claim-about-us-healt/
Nutshell: Bernie Sanders gets a "false" rating for saying that "we spend almost twice as much per capita on health care as do the people of any other country." The truth: We spend double the
average of other developed countries, but we only spend about 50% more per captia than the 2nd and 3rd biggest spenders, Norway and Switzerland.
Compare and contrast with:
http://www.politifact.com/truth-o-m...rubio-slams-dodd-frank-gets-his-numbers-wron/
Rubio gets a "mostly false" for saying, "Over 40 percent of small and mid-size banks that loan money to small businesses have been wiped out since Dodd-Frank has passed."
The truth: The
highest estimate is 16%, less than half of what Rubio said.
Rubio gets a "mostly false" presumably because there was something to his core point - a significant number of banks have failed even if it isn't anywhere near what he said.
One would think that Sanders' core point - that we spend massively more on healthcare per captia than any other country - is still valid. Why did Sanders' statement fare worse than Rubio's? Who knows.
I see this kind of thing all the time on politifact. The facts they present are highly useful, but the ratings may skew at times and it certainly isn't generally in a left direction. At best it's pretty much even.