Fbsbx.com

This domain name is owned by Facebook Inc. Source at https://gwhois.org/fbsbx.com+dns

According to Google on July 23, 2017 this domain 鈥渃ontains harmful content, including pages that: Contain suspicious or unknown software鈥? Source at https://www.google.com/transparencyreport/safebrowsing/diagnostic/index.html#url=fbsbx.com

This is a tough one and I marked it "suspicious" and "be cautious" for children.

I would leave it blocked in NoScript and uMatrix and temporarily allow only if needed.

There seems to be a couple xxxxxxx.fbsbx.com out there.

attachment.fbsbx.com seems to be for being able to view and download attachments in Facebook. Hence, the "be cautious"

ct-m-fbx.fbsbx.com seems to be associated with FB snooping on you. You might want to read:
https://gist.github.com/rainiera/b5d396a9ef3236b96864b0707bf54940
Hence, the "suspicious".

Belongs to facebook, seems to be connected to streaming videos on facebook. Beware though, Google says they hosted Malware:
http://www.google.com/safebrowsing/diagnostic?site=fbsbx.com

Owned by Facebook, Inc.