Acrostic Pi, a.k.a. @AcrosticPi, is a Twitter art piece by Jason McIntosh (@JmacDotOrg). Rather as its name suggests, it renders the number π as an acrostic, built out of other Twitter users' posts. It currently posts one digit (and therefore one retweet) every fifteen minutes or so. (It also updates its profile blurb to note which digit of π it has most recently posted.)
Here it is running. In its current iteration, the account has been posting new digits since π Approximation Day (July 22, a.k.a. 22 / 7), 2014.
A screenshot of its first few posts (as seen through the Tweetbot twitter client on Mac OS):
Outside of the ephemerality of the Twitter platform itself, this sculpture features an additional layer of impermanence in that any digit from its reckoning might vanish at any time. If the source of any of its retweets removes their post (or has their account hidden or deactivated), then the retweet vanishes forever, leaving a irreparable gap in this reciting of π. On an initial test-run of some 1,000 posts made over a three-week period, I saw a digit-loss rate of around one or two percent.
Therefore, please do not use this artwork as an accurate mathematical reference.
I did consider avoiding this issue by using quote-style RTs rather than actual retweets. But I decided that I like it better the way it is, even with the slow decay of numbers...
The source code is not available at present, but I do plan to share it in the fullness of time. (For another Twitter-sculpture by me that does feature public source, please see @EasternClock.)