  The luminosity function of Fast Radio Bursts (FRBs), defined as the event
rate per unit cosmic co-moving volume per unit luminosity, may help to reveal
the possible origins of FRBs and design the optimal searching strategy. With
the Bayesian modelling, we measure the FRB luminosity function using 46 known
FRBs. Our Bayesian framework self-consistently models the selection effects,
including the survey sensitivity, the telescope beam response, and the electron
distributions from Milky Way / the host galaxy / local environment of FRBs.
Different from the previous companion paper, we pay attention to the FRB event
rate density and model the event counts of FRB surveys based on the Poisson
statistics. Assuming a Schechter luminosity function form, we infer (at the 95%
confidence level) that the characteristic FRB event rate density at the upper
cut-off luminosity $L^*=2.9_{-1.7}^{+11.9}\times10^{44}\,\rm erg\, s^{-1}$ is
$\phi^*=339_{-313}^{+1074}\,\rm Gpc^{-3}\, yr^{-1}$, the power-law index is
$\alpha=-1.79_{-0.35}^{+0.31}$, and the lower cut-off luminosity is
$L_0\le9.1\times10^{41}\,\rm erg\, s^{-1}$. The event rate density of FRBs is
found to be $3.5_{-2.4}^{+5.7}\times10^4\,\rm Gpc^{-3}\, yr^{-1}$ above
$10^{42}\,\rm erg\, s^{-1}$, $5.0_{-2.3}^{+3.2}\times10^3\,\rm Gpc^{-3}\,
yr^{-1}$ above $10^{43}\,\rm erg\, s^{-1}$, and
$3.7_{-2.0}^{+3.5}\times10^2\,\rm Gpc^{-3}\, yr^{-1}$ above $10^{44}\,\rm erg\,
s^{-1}$. As a result, we find that, for searches conducted at 1.4 GHz, the
optimal diameter of single-dish radio telescopes to detect FRBs is 30-40 m. The
possible astrophysical implications of the measured event rate density are also
discussed in the current paper.
