Tweets about Carl Sagan, a personal voyage, on Radio 4, Saturday 11 April, 2009. Looks like the first tweet came at about 1530 (BST) from @giagia (presenter Brian Cox's partner, followed by over 2,700 people on Twitter). This triggered a flurry of retweeting, alerting a lot of non-UK listeners to the programme, among other things.
Tweeting peaked during the first transmission at 2000 on Saturday (about a dozen tweets mention the programme) but carried on until today (Tuesday) with another peak during the (abridged) second transmission on (Bank holiday) Monday afternoon.
For the Radio 4 blog.