The speed of light through a vacuum is about 300,000 km/s. The speed of light over a fiber optic cable is about 200,000 km/s. The distance between, say Singapore and New York, is about 15,000 km. If we pretend the cable runs all the way to the East Coast, this gets us to 0.075 seconds for light to travel that distance around the Earth over fiber.

So that's a theoretical minimum of 75 milliseconds additional lag in addition whatever network equipment exists in between you and your friends. Actually not too bad, even though we concede that the premise is slightly absurd.

If & only if Facebook sell access to capacity on the cable publically (They might just keep it for their internal use), and then if any of the providers that the gaming traffic uses start to use capacity on that cable.

However, fundamentally, even if fibre took the most direct route from your house, directly straight-line to the datacentre with the server in, and then straightline from there to your friends on the East Coast, the time taken to complete that journey and back is still going to be 150-200msec or so; so it won't be as snappy as if you all lived nearby, sadly.

What you want ( or what I really want ) is hollow cable running at near full speed of light. Currently fibre glass is only 2/3 of light.

This could potentially be 50ms latency saving from East Coast to SEA.

Unfortunately the tech is so far away it likely won't happen for another 10 years.