  Using the results of recent optical surveys we conclude that the {\it
non}-detection of quasars down to faint magnitudes implies a significant
flattening of the high redshift (z~6) optical active galactic nuclei (AGN)
luminosity function for M_{1450}>-24.7. We find that all the data are
consistent with a faint-end slope for the optical AGN luminosity function of
\beta=-2.2 and \beta=-2.8, at the 90% and 99% confidence level respectively,
flatter than the bright-end slope of \beta'~ -3.2. We also show that X-ray deep
surveys have probed even fainter magnitudes than the optical ones yielding more
significant constraints on the shallow faint-end slope of the optical
luminosity function. The inclusion of Type II AGN candidates, detected in the
Chandra deep fields, hints towards an higher normalization for the total AGN
luminosity function, if these sources lie at 5<z <6.5. We then discuss simple
theoretical models of AGN formation and evolution in the context of cold dark
matter cosmology. The comparison with the total AGN luminosity function favors
a redshift-dependent relation between black hole and dark matter halo masses of
the type M_BH~M_ halo^\alpha, with 1.3< \alpha< 1.7, compatible with
independent studies from statistical analysis and rotation curve measurements.
Finally we compute the quasar contribution to reionization to be <9% at z~6, up
to ~30% when integrated within 5.5< z<6.5, significantly smaller than that from
galaxies.
