While it looks like they never started the move over to AWS, the press release makes it sound like they do use some AWS services.

> In addition, Twitter will continue to use AWS services such as Amazon CloudFront (AWS’s fast content delivery network service that securely delivers data, videos, applications, and APIs with low latency and high transfer speeds to customers globally) and Amazon DynamoDB (AWS’s key-value database that delivers single-digit millisecond performance at any scale).