From a very lowbrow point of view, the question rests on the new ingredients string theory mandates not already embedded on either the Standard Model or General Relativity, namely supersymmetric partners and extra dimensions. The critique then goes that since we currently don't know the energy scales at which these things turn relevant (i.e. the mass or couplings of the susy fields or the radius of the extra dimensions) some have been tempted to trust string theory and claim that these energies are larger than the currently acessible through experiments. The fact that some people thought these energies should be probed firts in Tevtron and then LHC and now propose are evn higher amounts to the sentiment that if nothing is found one could always suppose it is in even higehr energies. A more detailed answer should adress the landscape issue, viz. Samuel Monnier's answer.  

On the issue of experimental confirmation, there are two alternatives: get lucky and see new particles in colliders/cosmic rays and check if they fit in some extension contained in string theory (not easy) or any precise cosmological prediction not explicable in current terms. For instance one could explain the value of the cosmological constant.

Just to make sure, the falsifiability question should not bother too much (especially mathematicians), there is a long literatutre discussing the fact that is a not very well defined condition, or that it does not adress the way science works (one could look up here for starters http://en.wikipedia.org/wiki/Falsifiability).