Effect of solutes on surface tension
Davies and Rideal1 published the following graph. This graph nicely describes the action of various solutes on a solution.



It's clearly visible that:


Addition of $\ce{MeOH}$ reduces the surface tension nonlinearly
Addition of $\ce{NaCl}$ increases the surface tension linearly, but by a small amount 
Addition of a surfactant such as $\ce{RSO3Na}$ decreases the surface tension drastically, up until Critical Micelle Concentration is reached.


Smith and Gillham2 present a more detailed graph of the effect 1-butanol has on the surface tension of water.



References:


Hansen, Robert S. “Interfacial Phenomena (Davies, J. T.; Rideal, E. K.).” Journal of Chemical Education, vol. 39, no. 7, 1962, p. A552. doi:10.1021/ed039pA552.
Smith, James E., and Robert W. Gillham. “Effects of Solute Concentration-Dependent Surface Tension on Unsaturated Flow: Laboratory Sand Column Experiments.” Water Resources Research, vol. 35, no. 4, 1999, pp. 973–82. doi:10.1029/1998WR900106.