Through a managed hosting agreement, NTT America handles the heavy lifting for Twitter's infrastructure. Back in June, the COO of NTT America was quoted as saying "traffic generated by Twitter is getting so big, it’s basically eating up a lot of our data center network resources, especially the segment where Twitter is hosted." Around that time, critical infrastructure upgrades had to be postponed when Twitter became a vital tool during the Iran elections.
The new facility in Santa Clara, CA is decked out with redundant power supplies and water cooling systems, advanced climate controls and a direct pipeline to NTT America's Tier 1 backbone. The increased reliability afforded to Twitter by the additional facility probably won't be able to help thwart incidents like the Denial of Service attacks that led to the service's downtime recently, but is sure to help balance network traffic generated not only by the microblogging service but by other web sites co-located at NTT America's facilities. The Silicon Valley is obviously a hot corridor for internet traffic and there should be a number of sites that stand to benefit from relieved traffic pressure from Twitter.