  The $f(R)$ gravity models formulated in Einstein conformal frame are
equivalent to Einstein gravity together with a minimally coupled scalar field.
The scalar field couples with the matter sector and the coupling term is given
by the conformal factor. We apply the holographic principle to such an
interacting model in spatially flat and curved universes. We show that the
model leads to a constant ratio of energy densities of dark matter to dark
energy in a spatially flat universe. In a spatially curved universe, the ratio
is not a constant and the evolution seems to be model-dependent. However, we
argue that any cosmologically viable $f(R)$ model can lead to a nearly constant
ratio of energy densities and therefore alleviate the coincidence problem.
