Fbsubnet+l

The terminal beeped softly.

In the context of web analytics and social media tracking, "fb" stands for Facebook, and often refers to the l.facebook.com fbsubnet+l

She wasn’t sure if the system was addressing her—or if Kael had coded that message into the subnet years ago, waiting for her to finally arrive. The terminal beeped softly

(the number of bits in the subnet mask), it determines how many addresses you have: "fb" stands for Facebook