I get the idea that TDM and FDM can be used in circuit switched networks, but what about packet switch ones? Can FDM be used?
Time-division multiplex and frequency-division multiplex are different schemes to multiplex a single physical line for different services or flow directions.
With packet-switched networks, the multiplex for different services is already integrated, so there's much less need for additional multiplexing. Wi-Fi uses (a kind of) FDM to allow multiple channels. These channels can be used independently or they may be bonded to increase the bandwidth of a single link. On copper, FDM and TDM aren't commonly used in LANs (due to baseband signaling) but xDSL and DOCSIS use FDM (they're not really packet-switched though). You could also argue that early Ethernet's CSMA/CD is a dynamic kind of TDM.
Wavelength-division multiplexing is in common use on optical fiber to extend their capacity. WDM is also used to multiplex a single fiber strand for bidirectional use (e.g. 1000BASE-BX and 10GBASE-BR).