Why am I seeing keep-alive requests for api.facebook.com in Fiddler without a request to api.facebook.com?

Well, the question is a bit of a lie. My web page does make https: requests to api.facebook.com. But here's what's puzzling me. I open up Fiddler and load my page. My page does make a request to api.facebook.com at some point. I wait until my page is making no requests at all. I refresh my page, and well before my page hits the code to make a request to api.facebook.com, I see in Fiddler a keep-alive request to api.fa开发者_JAVA技巧cebook.com:443; the reply has the server certificate information in it. In fact, I will often see a couple of these requests after I tell the browser to reload the page, but before the page itself is fetched (according to Fiddler). If I test with Chrome, I don't see the request in Chrome's network console. If I clear cache and cookies and refresh, I don't see the keep-alive request until I make the request to api.facebook.com.

I appreciate any help.