Star topology is very common; chances are any company you can think of (big or small) is using a star in their LANs somewhere within the company.
Star topology.
Star Topology
The most common LAN topology is that of a "star." In a star topology, each computer, or "node", is connected to a central hub. This is more reliable than a more classical "ring" topology, because a node failing will not bring down the entire network. A bus topology is arguably more reliable, but has poorer performance.
The "star" topology is the most common.
No one knows, since this is not published information. Companies are not likely to publish anything about their network topology, since it may be considered a security risk.
Star topology
Star topology is one of the oldest topology
Star Topology, where Hubs can act as repeaters.
bus topology.
cat5, cat6, utp, stp cables etc.
Scalability and reliabillty of star topology makes it the best, easy to remove or add a device.