Creating backlinks will get you nowhere. You need to earn backlinks, by having great content that others will choose to link to. Search engines regard links which you place yourself as all but worthless.
That will have no effect on SEO. Most social sites follow Google's guidelines and mark user-submitted links as nofollow.
Likewise, that will have no effect, as high quality blogs will also follow Google's guidelines on user-submitted links. (They may well also remove comments deemed to be Spam, placed only as an excuse to liink-drop.)
Creating links with the intention of manipulating search results is a violation of search engine guidelines, and may result in your site being penalised.