In this lj_biz
car crash discussion, halfjack wondered what sort of effect the removal of invite
codes had on the rate of account creation at LiveJournal. I did a quick
gnuplot chart of the result, which was pretty striking.
I decided to do a better chart as a quick project to learn gnuplot a bit better, and ended up charting account creation rate and userbase for what I believe are the five largest LiveJournal-based journal sites: LiveJournal, DeadJournal, GreatestJournal, Blurty, and uJournal.
So invite codes definitely have an effect -- you can see the dramatic changes when LiveJournal introduced and removed invite codes, and smaller but still striking changes on uJournal. (I'm not sure what caused the drop in DeadJournal account creation halfway through 2002; I remember a dataloss issue there around then, but that shouldn't affect new users. Does anyone know when DeadJournal introduced invite codes?)
One thing to keep in mind when reading the above chart is that LiveJournal's decision to abandon invite codes coincided exactly (and intentionally) with the first screenings of the LiveJournal commercials. Some of the peak was obviously the result of moviegoers seeing those commercials, but the level has stayed pretty high since then.
I decided that one huge inline chart was enough, but you can also see the chart of total users per site in the same time period (based on the number of new accounts, which is the only archival data available). You can also see the gnuplot program which created the charts.