We all know:
FR-Layer 2 address - DLCI
MPLS-Layer 2 address - Label
Ethernet-Layer 2 address - MAC Address.
What is the layer 2 address used by PPP & HDLC?
There is no L2 address for those. These are point to point technologies, meaning there is only one device on the other side. Therefore, there is no need for L2 addressing.