As for the reasons why you may think foreigners are more skilled than locals there is this explanation that fits best into your use case: if you look at immigrants in context of their source society you will see, that they are either over- or undereducated. The ones in the middle usually have more to lose and less to gain from immigrating. Low skilled have very little to lose and very skilled usually migrate because they have better options in other countries and not only financially. In other words, if you were looking you would have found locals of the same skill level as immigrants are but immigrants asking for a job in IT are usually filtered by the migration effort that is huge and has to pay off so mostly well skilled take that risk. This is a simplification of real process but as long as people do not try to flee natural disaster of Pol Pot proportions they usually form the diamond shape when put on mass/skill diagram.
As for lowering of numbers of skilled workers there are many reasons I would think. USA was once major magnet because the rest of the world was so underdeveloped. This has changed. Then more restrictive environment means that to get in, you have to do more or get there illegally - such conditions are putting skilled ones off naturally as it increases risk without increasing of pay off. But there are other reasons too. US society is better known to people now and not from better side. If decision to immigrate is based on information and calculation instead of myths and herd instincts, then there is ever less incentive: US society is more and more seen in perspective of great divide between have and havenots, there are riots, militarized police seems to be shooting people at random and people are shooting other people, tensions in society seem to be fixed so people in say Fergusson buy guns instead of trying to address the problems, add to this war on drugs, massive prison population and what is important for people like me who would immigrate with family: basic social safety nets are somewhat lacking. There is also another aspect - US is not seen as a good country anymore or not by majority. Take Germany - conquered by (among others) US military power and turned into ashes in a process yet people loved US after a while. This has stopped and now they are more critical but there are plenty of places in the world where people see US as only good to get some goodies AND a great danger to own society too. I do not know how many people use such arguments not to migrate to US but I would imagine the number became significant of late.
Plus today there are more jobs which you can do from home. You can do some of those jobs from home - income may be lower but risk is lower too.
As for your /. argument - I think you exaggerate hugely the influence this site has on anything. I know maybe 2 or 3 guys who know /.
If this site had any influence on general population anywhere we would see libertarian economic policy at least in US, energy needs covered by 4th generation of nuclear reactors and NSA and religion banned from the system their proponents sent to re-education camps in Syberia. maybe even some good examples could have been found of such geek based society but we would not know because this is not happening.
