Cool idea, but likely to break or be compromised if ever used in any significant capacity. No one has explored attack vectors yet.
I very much agree with these statements. This can not possibly scale even up to smallish numbers. Actually its the same reason exchanges can not be distributed. The blockchain can't be used for time-ordered messaging. Transactions in the chain are not ordered. The order of transactions in one block does not matter. However, order of messages in communication does matter. If in a thread one post comes later than another but refers to ts hat post, order is not preserved. So a conversation (i.e. message B is a reply to message A) is not possible, because order is needed.
I think there is generally not a good understanding of what can be done with the blockchain in principle. The most interesting application by far is namecoin. The reason is that the cycle of messages is much longer. You reserve a name and this stays constant for say 12 months. I find it also interesting in that a domain name really is a link to a server, operated by a person or organization. If you think about it a corporation is really not much else. Twitter is a set of people tied to the services of Twitter Inc, a completely fictitious entity. There is no reason we could have much better legal frameworks. For example most multi-nationals avoid taxes completely with various schemes, corrupt governments, hand over information to power groups etc.etc.