I think they’re 2-fold:
– the social aspect of blogging (interacting with people) is challenged by Facebook. it’s the “I don’t use my blog anymore. All the people I’m trying to reach are on Facebook.” argument.
– the content creation aspect of blogging face a new distribution system which relies on curation. Through Twitter, Facebook or new curation platforms like Scoop.it, readers click on what curators have shared rather than simply follow blogs via RSS or any other forms of direct subscription. While historically readers were loyal to a media brand, they now have much more options and are influenced by curators to decide what they read.
Ironically, this latter is a challenge that blogs share with traditional media.