We first discuss the occurrence of an ergodicity breaking energy threshold in anisotropic spin systems and we show that this threshold determines magnetic reversal times. We called this threshold Topological Non-connectivity Threshold (TNT). Below this threshold the system is non ergodic, which is: it cannot explore the available constant energy surface. This implies that in an isolated system the magnetization cannot reverse its sign below the TNT, while above the TNT we found a power law divergence of the magnetic reversal times at the TNT. In case of long range interaction among the spins, the disconnected energy portion determined by the TNT remains finite, even in the large particle number limit. On the other hand for nanoscale structures, the TNT has important consequences both for short and long range interacting spins. When the system is in contact with an heat bath, the magnetization can reverse its sign even below the TNT, but this threshold has still important consequences on magnetization reversal times: this threshold represents an effective energy barrier for magnetic reversal, so that for average thermal energy below the TNT, the reversal times depends exponentially on this energy barrier. In order to understand whether the TNT can determine a ferromagnetic behaviour in nanoscale structures, due to long magnetic reversal times, we analyze realistic long range interactions, such as the dipole interaction and the RKKY interaction, which might be responsible for the ferromagnetic behaviour of diluted magnetic semiconductors, with a Curie temperature around 100K and for the room temperature ferromagnetic behaviour of diluted ferromagnetic oxides, having Curie temperatures far in excess of 300K.