didn't miss it, doesn't work.
Economics: at a max of 21 blobs per block (every 12s), you'd need an intermediary to compact posts from users to even attain the claimed 498 million messages a day max. Assuming somehow the fee market didn't fluctuate and all 21 blobs could be happily filled by social blobs, we're talking a cost of .15 ETH a day just to post the data ($11k a month). However...
This assumption includes a 240 byte message size limit, which is under short and long cast limits, meaning we're already off target on how many messages could be posted. Worse, the blob market depends on the max not being regularly hit, and fees will rescale accordingly, resulting in a growing cost every block. Worse still, this isn't the only thing posting blobs, which further increases the excess blob gas usage going well over target, raising the cost.
-- {no channel}