I have found many Feeder sites like hubpages.com, xanga.com, zimbio.com, wetpaint.com, bukisa.com, xomba.com These sites fetch articles from RSS and auto publish. I want to know if these sites help to increase Search Engine visibility or this is called spamming?
They work as long as you don't get caught. Most scrape farms tend to get nuked sooner or later if they only used scraped content. Sites you mentioned have a lot of unique user generated content
hubpages.com, xanga.com, zimbio.com, wetpaint.com, bukisa.com don't fetch articles from RSS and auto publish. They publish user generated content and sites like hubpages do not allow duplicate content and are even moderated. Also, most of them are nofollow. This means that building link on them will have no impact in search engine ranking. However as they are high traffic sites, when you post quality articles on these sites, you can have traffic increase which will benefit to your site in long term.