Sina Weibo is 50-60% of Sina's valuation – Is it too High ??

Recently, I have been talking with an investment bank analyst about Sina.  I asked how he valued Sina’s Weibo (its miniblog or microblog services).  He said he will give its portal business 30 times PE or about US$30 per share.  Its wireless business about US$2.  And another US$5-6 for its other business (apart from Weibo).  It has US$13 in cash per share.  That means excluding Weibo, Sina should worth about US$50.  Currently, Sina is trading at about US$117.  Therefore, investors are giving 57% of Sina’s valuation to its Weibo alone, or about US$ 4 billion dollar!!

Is it too much for something has yet to prove its business model?? Revenue from Weibo is still tiny at the moment.