Facebook's scraper doesn't need to read the full page, so they only request part of it by sending a Range header. However, your server seems to be responding incorrectly to that request and returning a 403 error code. You need to check your server and make sure it handles the range header correctly.
To see this in action, try this CURL command:
curl -H "Range: bytes=0-524287" http://www.miswag.net
This is the response I got:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access /
on this server.</p>
<p>Additionally, a 404 Not Found
error was encountered while trying to use an ErrorDocument to handle the request.</p>
</body></html>