Ask HN: Why a surge in direct traffic after running a FB ad?

Any ideas why we'd start receiving abnormal direct traffic to our site after testing out a Facebook ad for one day?

Our traffic isn't huge, but on a typical day we receive around 300 direct hits with a 32% bounce rate.

After running the Facebook ad for 24 hours (it then expired), our direct traffic went to around 800 hits with an 86% bounce rate.

It appears that all of the new direct traffic is from the US, but it's from pretty obscure locations.

Any ideas?

Thanks, T