Now going into the code part
<span style=3D""><img = src=3D"https://www.facebook.com/email_open_log_pic.php?mid=3D5bd7580G5af3c= 45d6bc9G459583G96" style=3D"border:0;width:1px;height:1px;" /> <bgsound = src=3D"https://www.facebook.com/email_open_log_pic.php?mid=3D5bd7580G5af3c= 45d6bc9G459583G96&s=3Da" volume=3D"-10000"/></span>
Here the first part is legitimate they are using img tag for image, but the second part bgsound don't know what it really is. In HTML it is used for background sound.
If you want to check you will need to enable HTML and show images in mail service in my case it is Gmail.
1. Open an email send to you by Facebook.
2. Now click on the more button.
3. Now in the drop-down menu click on Show Original.
Another tab will be opened now search for bgsound, you will get the snippet down below.
Although, this tactics is used by many services. This is not termed wrong but your privacy is monitored. Basically Facebook is doing email marketing.