So your inbound load is 7000 tweets/sec or roughly 250 MBit/s (assuming 4k per tweet). Then you fan that out to (assuming) 20 append-only mailboxes on average.
Perhaps my assumptions are far off, but I'm only arriving at a couple GBit/s here and a low two digit number of terabytes/storage per year.
This sounds like "a couple racks" to me, not like "a couple datacenters".
http://www.slideshare.net/nkallen/q-con-3770885
Now, at the time they were doing peak 2000 tweet/s. The fan-out was 1.2M deliveries a second... So if we go with the current 600:1 ratio at 7000/s, that's about 4.2M/s. I actually know it's much higher now since I work there but other things to consider is we have a large data warehouse, search, API, pipelines to external parties for the firehose, logging at terabytes an hour, in-house metric collection doing 3M writes/s, etc.
http://www.scribd.com/doc/59830692/Cassandra-at-Twitter
It add's up very fast.