Jacob Weisberg, chairman of the Slate Group, tweeted an impressive statistic Tuesday afternoon: “Slate’s biggest traffic month ever is ending with a sprint to 100m PVs.”

The final tally for May was 101 million pageviews, in fact, according to Omniture data Slate editor David Plotz shared with us. That number rises to 106 million if you include traffic to Slate’s mobile site and apps. Slate.com attracted 15 million unique visitors, also a record, and 16 million uniques if you count the mobile users. Plotz said that’s almost double the numbers of a year ago.

“A lot of people had a good month because of Osama bin Laden,” Plotz said. “I can’t say for sure that we benefited more than other people…but the first few weeks of the month were outrageous.”

The magazine’s No. 1 story last month was not about bin Laden per se, but related: Cats of War, a slideshow revealing “the Pentagon’s top-secret feline special-operations program.” That cunning bit of lolcattery brought in a cool 3.7 million pageviews, Plotz said. Other top stories included William Saletan’s piece arguing Osama bin Laden’s porn stash might have harmed his image more than his record of mass murder, as well as Annie Lowrey’s explainer on who gets the FBI’s $25 million OBL bounty (no one).

Slate has made a lot of changes that are technical, not editorial, to grow its traffic over the past six months — starting around the time the New York Observer called it out for being “as technologically sophisticated as Chitty Chitty Bang Bang.”

The magazine has since hired a new technology director, Dan Check, to improve search engine optimization; and an “innovations editor,” Katherine Goldstein, who managed the relaunch of The Slatest as a HuffPost-y news blog. The Slatest saw a 30 percent jump in pageviews from April to May, from 1.8 million to 2.6 million. Inbound traffic from Facebook and Twitter grew 600 percent over the year before, Plotz said, and editors are continuing to experiment with social media. Slate’s popular Twitter feed has become more conversational, less institutional, and a touch political at times.

Slate is still stuck with a gray-haired content management system that generates numbers instead of words for URLs, which Google looks upon unfavorably. Plotz said that will change soon: The Slatest is using a new CMS as a pilot for the rest of the site.

It was also big month for Slate’s sister publication, Foreign Policy. Editor Susan Glasser said 2.9 million visitors viewed more than 21 million pages in May, a record. Again, Osama bin Laden drove the traffic — but Rebecca Frankel’s photo essay on war dogs (upon which Slate’s war cats slideshow was based!) was FP’s “biggest viral Internet hit ever,” generating about 8 million pageviews. Even without the war dogs, Glasser said, traffic is double that of a year ago.

Slate’s competitors at The Atlantic cleaned up, too, reports Business Insider. That newly web-focused magazine recorded 10 million unique visitors for May, a record, about twice as much traffic as at this time last year, even with the departure of Andrew Sullivan.

How did your news site do traffic-wise in May? Share your data in the comments.