Why pentoses are absorbed by facilitated diffusion in intestine? Wouldn't it be futile because , when the pentose concentration is lower in intestine, the pentoses would diffuse back to the intestinal lumen? So does our body looses them? If not then what is preventing this backflow?
First, pentoses aren't a major source of nutrition, at least for humans. That said, pentose absorption works the same way a lot of nutrient absorption works: the "magic" of metabolism, with respect to diffusion.
Pentoses leave the intestinal lumen, entering the intestinal cells, where their concentration is lower. From there, they again travel via facilitated diffusion into the bloodstream, again because their concentration is lower in the bloodstream.
From there, we can expect one of two outcomes: either an equilibrium will eventually be reached(i.e., lumen==intestine==blood), or the bloodstream pentoses need to be metabolized into something else. Indeed, certain pentoses (specifically D-ribose; I'm not sure if others can end up in this pathway as well but it is a very common pentose) can be metabolized in the pentose phosphate pathway for anabolic purposes. Others may end up in proteoglycans.
Regardless of the metabolic pathway, the effect on the process you are wondering about is the same: the metabolites no longer "count" in terms of concentrations for diffusion. Because membranes are not permeable to everything, if they are not permeable to the products of metabolism there is no way for those products to "diffuse backwards."