Hi Rohan,
Ethernet is more often used nowadays for the WAN. I wrote a bit about this in this lesson:
For MPLS, it doesn’t matter that much what the underlying network is. MPLS does support transport of L2 frames, including frame-relay. This allows you to keep your current frame-relay routers on the customer side but replace the frame-relay provider with an MPLS network. Here’s a quick example:
In the example above, the HQ and Branch routers have Ethernet interfaces but it’s also possible to use serial interfaces with frame-relay encapsulation on those routers.