According to press reports Chinese e-commerce conglomerate Alibaba Group has not reached an agreement on buying a stake in Chinese Internet company Sina Weibo.   A disagreement on valuation is rumored to be the principal reason hindering the two parties from reaching a deal.  Previous sources have said that Alibaba will have a 15-20 percent stake in Sina Weibo, which will value the Twitter-like service at $2-3 billion.