Thank you guys for the response. Last week I started adding my site to social bookmarking services as well as to web directories and started making natural blog comments. 3 days ago my site jumped from the 432th position to the 2nd on the main keyword according to the WP Google Rankings and it remains so. Thanks to everyone.
Before to buy backlinks you must consider: 1) the age of your site/blog 2) how many pages you have 3) quality of your content Sometime buying backlinks can hurt your business... if your content is younger.
The backlinking service you may have gotten is actually a service that sends floods of links for your blog - which is not good of course and looks like spamming by Google. If it hurts your blog, that may be the best possible reason. Next time, it would be better if you just build backlinks on your own than just getting a backlink service.
If your backlinks are on really spammy or flagged websites that could perhaps hurt your site. Also if the backlinking was done in very large numbers in a very little time that is also a big red flag. The best way to get your backlinks done is organically and naturally over time.
You have to be careful about just buying any backlinks, especially if you have an established ranking.