Werner Vogels talks about the scalability of feeds and aggregators, which runs into two problems, more interesting feeds than one person can handle and more subscribers than the feed provider can handle:

The increase in the number of feeds will leave many users frustrated, as there is a limit to the number feeds one can scan and read. Current numbers suggest that readers can handle 150-200 feeds without too much stress. But users will want to read more and more as new interesting feeds become available and they run into the limitations of the metaphor of current aggregator applications. The current central abstract of aggregators is that of a feed, and there is a limit to how many individual feeds one can actually handle. Aggregators will need to find ways in which the users can be subscribed to a select set of feeds because they want to read everything that comes from these feeds, but also subscribe to a much larger set of publishers for which the feed abstraction may not be the right metaphor. Aggregation, fusion and selection at the information item level instead of at the feed level seems to be a first abstractions to investigation. Advances in how users can specify what information they would like to see, will be enablers for scalability at the human level.

...via The Shifted Librarian