There's a good post from John Battelle, who has been very impressive with deep, insightful analysis lately, on Twitter's decision on focusing on monetizing the interest graph.  This means that Twitter will be very supportive of ventures who build on Twitter which enhance the interest graph, but antagonistic agains those who are, in parallel, destinations with their own (interest-based) community ambitions.  John points out Tumblr and Instagram as recent victims.

I don't think I can blame Twitter. It now has 100m+ actice users, shareholders who have come in at massive valuations, and a responsibility to start filling underneath the financial assumptions behind thoes investments.  I also think Twitter has an equally viable option to remain a true, neutral platform.  But that takes longer.  Look at what happened to shares of Facebook. Platform businesses take a long time to effectively monetize.  If the internet was a private company, it would be laden with much more friction today.