  In light of recent indications for a large population of obscured active
galactic nuclei (AGNs), we revisit the mean radiation efficiency from accretion
onto supermassive black holes (SMBHs), epsilon, applying a bayesian approach.
We use the integrated comoving energy density emitted by AGNs and compare it to
the mass density of local SMBHs. When considering only optically-selected
unobscured AGNs, we derive log_10[epsilon]=-1.77+0.16-0.11 or
epsilon=1.7+0.8-0.4%. Using the AGNs selected using hard X-rays, which include
unabsorbed and Compton-thin absorbed AGNs, we find
log_10[epsilon]=-1.20+0.15-0.10 or epsilon=6.4+2.6-1.3%. Using
optically-selected AGNs, and correcting for the obscured population, we inferr
log_10[epsilon]=-1.17+0.11-0.08 or epsilon=6.7+1.9-1.1%, which we consider our
best estimate. We also repeat this calculation for intrinsically luminous AGNs
(M_B<-23, quasars), comparing to the SMBH mass density in local elliptical
galaxies, and find log_10[epsilon]=-1.27+0.15-0.11 or epsilon=5.4+2.2-1.2%. We
discuss how our results can be used to estimate the mean spin of accreting
SMBHs, both assuming the standard thin-disk model of accretion onto black holes
and also comparing to more recent simulations. Our results can rule out
maximally rotating SMBHs (<a>=0.998 Gm_bh/c^2) at the >=98% confidence level,
as well as high rotation values (<a> >=0.9 Gm_bh/c^2) with >=92% confidence.
Our preferred values of <a> are ~0.25-0.60 Gm_bh/c^2, although even these might
be overestimated. Hence, we find that on average, SMBHs are not rapidly
spinning during accretion. Finally, using an independent measurement of
Eddington ratios, we estimate the SMBH e-folding time for the brightest AGNs
(quasars) to be <tau> =100+151-60 Myr.
