The authors in the paper analyze phenomena of Social Internet Portals and main reasons of their development. The research is based on theory of sociology, management and marketing. Main problems what are common for both – developers and users of Social Internet Networks are described in the article. Authors offer their classification of Social Internet Portals. There is insight in theoretical and practical aspects of Social Internet Portals for entrepreneurship and marketing described in the article. The authors analyze possibilities to use Social Internet Portals and benefit form this usage form business standpoint. Very special attention is paid to usage of Social Internet Portals for communication with potential customers and targeting of main markets. Authors analyzed influence of Social Internet Portals to main activities of business, including rising of competitiveness and rivalry within industries. Research is devoted to answer on question – how does Social Internet Portals influence sales, marketing, communication and how does entrepreneur can use these networks for increasing business efficiency and effectiveness. During research authors analyzed Social Internet Portal www.twiiter.com and gives real examples for positive and negative communications via Social Internet Portals. Authors concludes that Social Internet Portals are important tool for rising competitiveness of a business, unfortunately there are a lack of entrepreneurs’ knowledge and skills for efficient usage of this communication tool. Also the lack of computer literacy is important factor what influences the results of communication via Social Internet Portals.