UPDATE: The info below is sort of correct, but per Jon's comment below, my intended message is incorrect. The truth is that Slashdot (which is really Slashdot.org, not .com) is #853, putting it still ahead of Digg.
According to Netcraft, Digg is the 1,943-ranked site in terms of traffic, compared to a 19,593 ranking for Slashdot. While it's true that Digg is still ranked higher for Alexa, that's just a sign that Digg users don't download and install the Alexa toolbar.
Alexa does rank more sites than Netcraft, but Netcraft is more accurate on the sites it does rank. For example, Alexa ranks a Disney Antenna Ball Collection site 4,572,690th, while Netcraft ranks the site's traffic as "unknown".