There seems to be a general misconception when it comes to auto-blogs.
The 'auto-' part refers to the content, not the traffic. Traffic must be generated the same way as any other site - SEO, backlinks, forum & article marketing, etc.
As far as content goes, an autoblog without any original content, in general, will not do as well one with, say, 5 original articles, although it does depend on the niche. Auto-posting scraped content, bookmarking and pinging will only get you so far.
WP-robot is one of the most popular auto-posters.
Free plugins? Try wp-fetch.
There are loads of other auto-posting plugins available on this site.
Hope this helps.