The dataset with #ows hashtag contains approximately 5.3 million tweets contributed by 650K users between October 2011 and December 2011. The following graph shows the number of new users who posted a tweet on a given day (i.e., the users who hadn’t posted before that day):
The graph of the cumulative number of new users (i.e., number of unique users who had posted up until the given day) is below.
The final dataset and charts are here: https://docs.google.com/spreadsheet/ccc?key=0Ash6mAvb0VyZdER3RDFaY0JFNlVWMnlqQXZGSExaRVE
Details of the analyses and scripts are below: