  We assess the influence of massive black hole (MBH) ejections from galaxy
centres, due to the gravitational radiation recoil, along the cosmic merger
history of the MBH population. We discuss the 'danger' of the recoil for MBHs
as a function of different MBH spin/orbit configurations and of the host halo
cosmic bias, and on how that reflects on the 'occupation fraction' of MBHs. We
assess ejection probabilities for mergers occurring in a gas-poor environment,
where the MBH binary coalescence is driven by stellar dynamical processes, and
the spin/orbit configuration is expected to be isotropically distributed. We
contrast this case with the 'aligned' case. The latter is the most realistic
situation for 'wet', gas-rich mergers, which are the expectation for
high-redshift galaxies. We find that if all halos at z>5-7 host a MBH, the
probability of the Milky Way (or similar size galaxy) to host a MBH today is
less than 50%, unless MBHs form continuously in galaxies. The 'occupation
fraction' of MBHs, intimately related to halo bias and MBH formation
efficiency, plays a crucial role in increasing the retention fraction. Small
halos, with shallow potential wells and low escape velocities, have a high
ejection probability, but the MBH merger rate is very low along their galaxy
formation merger hierarchy: MBH formation processes are likely inefficient in
such shallow potential wells. Recoils can decrease the overall frequency of
MBHs in small galaxies to ~60%, while they have little effect on the frequency
of MBHs in large galaxies (at most a 20% effect).
