Currently 241 exoplanets. [http://en.wikipedia.org/wiki/Gliese_581_c/URL] A few are terrestrial such as Gliese c, d at 5x and 8x earth mass; sufficient mass to be geologically active, and hence a magnetosphere, diverting a stellar wind, and thus preventing eroding of any atmosphere and oceans. An O_2 molecular spectographic signature from a exoplanet atmosphere is a sign of photosynthesis. Earth has had such oxygen signature for 2 billion years; while just technological for say 200 yrs; that is 1 part in 10^7 (200/2 x 10^9). So if one divided 2 billion years into bins of 200 years each, one would require 10^7 such bins until one discovered an exoplanet with an oxygen signature, that had a technological civilization. This is assuming that our earth history is typical. Re-expressed: one would need a database of 10 million exoplanets with an atmosphere oxygen signature before finding a technological civilization! How many terrestrial planets are there in a galaxy of about 10^11 stars? The pattern for exoplanets seems to be tending towards abundance. If only 1/10 of stars had 1 terrestrial, then 10^10 terrestrial exoplanets. Would and O_2 signature for 1/1000 exoplanets seem too high? If not, then aren't there approximately 1000 (10^10/10^7) technological civilizations in our galaxy?