Lightning network payments can be routed through other users/nodes. So you only need one "on chain" transaction to then be able to send those funds back and forth to any other user in the network.

eg: Alice opens a channel with Bob. Charlie opens a channel with Bob. Alice can send payments to Charlie by having them routed through Bob (in a trustless way using Hashed Timelock Contracts).

There are valid concerns around what the topology looks like and how efficient it is to find optimal routes through a large network. These are the problems the developers are working on though.