I know many people whose Google Adsense account got blocked when they try to get traffic through Paid to Click (PTC) and traffic exchange sites. Googles considers it as inorganic traffic and I think they are quite right too. It is explained in their policy guidelines. In PTCs actually there is no value addition, the members just complete their daily quota of viewing ads to earn money. They are least bothered about what is being advertised there. That is why no big companies opt for PTCs to market their products. In traffic exchange sites, there are no buyers and you can see only sellers there. Members are busy selling/advertising their products and not interested to see other ads. They just view ads to earn ad credits so that they can use it to advertise their sites. Both are quite a mechanical process and does not involve any sincere efforts. Google loves organic traffic and hence support sites like forums where people interact each other and take part in meaningful discussions. Same is the case with article writing sites too.