Slate's tech-advice column answers your questions about marketing on social networks, Gmail's deletion policy, and more.

If you've got a tech problem you want solved, please send a note to farhad.manjoo@slate.com, with "I've got a tech question!" as the subject line. (Your question may be edited.)

Dear Farhad,
How does Facebook pick which ads to display? I'm gay, out, and proud, but my sexual orientation isn't listed on my Facebook page. However, Atlantis' gay-cruise ads still appear when I'm browsing. Has the site somehow figured out that I'm gay?

—Not Happy With Facebook Ads

Dear Not Happy,
According to a Facebook spokesman, you've most probably been swept up in an ad targeted to a very broad group. When companies advertise on Facebook, they're allowed to choose a range of demographic characteristics that determine which people see their ads. It's possible that Atlantis didn't choose to limit its ads just to gay people but, say, to all single men under 40 who live near San Francisco. This way the company gets to people like you—folks who aren't out on Facebook but who might still be in the gay-cruise demographic.

The Facebook rep added a couple other points: Ads aren't selected based on groups you've joined or based on your friends. You weren't shown the gay-cruise ad because your friends are gay or because you became a fan of the group "No on Prop 8," for instance.

But there is one caveat: If a friend of yours presses "Like" on an ad, Facebook will show you the ad, too, plus a note saying which of your friends liked it. The company also uses the "Like" feature to determine which ads to show you in the future.

—Farhad

Dear Farhad,
When you delete an e-mail or any other document from a Google account, is it gone forever? I've been wondering about this as I've become an active user of Gmail, Google Docs, Google Voice, and Google Sites. Google recently provided e-mails to prosecutors looking to convict a Bear Stearns trader of malfeasance—even though the trader had shut down his Google account. I know that if I click "Delete," I'll never see my e-mail again. But will Google still have it—not to mention any government agency that comes knocking?

—Google Knows What I Did Last Summer

Dear Google Knows,
You're right that Google doesn't actually delete your e-mail when you click "Delete." According to a spokeswoman, e-mail remains on Google's servers for 60 days after you've trashed it. Other Google services have a similar lag. It takes up to 30 days for a deleted document to get off Google Docs, 60 days for a deleted picture to vanish from Picasa Web Albums, and 90 days for deleted voice mail to be freed from Google Voice. Google, like all Web companies, does comply with court orders to produce user information, though it also has a history of fighting egregious requests. In 2006, Google managed to get a court to limit significantly the scope of a Justice Department subpoena requesting two months' worth of search data from Google users.