Of course, the reason Snopes.com is more often accused of bias by conservatives than by liberals is that facts have a notoriously liberal bias.
On the snopes message board, most of the charges of media bias I recall come from Ramblin' Dave, who charged mainstream media with a conservative or pro-Republican bias. Is this evidence that facts have a conservative bias? I don't think so.
There are many possibilities other than that facts have a liberal bias.
It hasn't even been established that the people who make claims of liberal bias actually have read much on the snopes site. More likely, they are captives of a false UL about a UL-focused web site.