Here’s a look at Experimonth: Mood registrations over the four days we recruited participants.
We announced the project on Monday at 10am through updates to Facebook and Twitter, hoping to get 40 non-museum staff registrations by Friday. Right away were invited to do guest posts for Quantified Self and Scientific American (thanks to Nina, Paul, Bora and Anton for their parts in that).
We had a couple days to prepare for the potential influx of attention, so we made calls to and tested our service providers and readied the site for a pay option of $6.47 to participate (we didn’t budget for more than 200 users and that’s the cost per).
Here’s how the community grew one registration at a time over the four days:

Day One: We announced the project at 10am and had a bunch of new users right away. Registration was steady throughout the day, Facebook referrals (204) were three times that of Twitter (65). Our highest day by one registration, we added 53 participants. We had 17 participants (beta testers and staff) prior to announcing the project.

Day Two: We didn’t do anything to promote the project on day two in preparation for the potential number of signups through QS and SciAm referrals. Registrations were slow in the middle of the night and slow, but steady throughout the day. We added 19 new participants.

Day Three: We began the day by load testing our server to make sure we could handle a lot of traffic if it came our way. This testing inadvertently broke our registration form and when I updated Twitter and Facebook for locals to signup before the 200 free registrations were filled, the site was down and we didn’t know it. You can see the window of dotlessness on the chart. We fixed the form and told folks of our error, but I’m sure some registrations were lost.
Around 3pm, both the QS and SciAm posts went live, but traffic did not balloon like we anticipated. We ended up with 53 referrals (visits, not registration) from QS and 36 from SciAm over two days. We added 42 more participants on day three.

Day Four: We updated Twitter and Facebook with “there are still free spots” messaging and got a pretty good bump. I’m unsure if there’s any significance to the gap between 2-4, but the traffic of the site was a little low then, too. We added 52 participants.
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At the end of the four days, we ended up with 166 new registrations. Our total number of participants is 183, 160 of which are not museum staff (four times my original goal). The site’s traffic analytics show 703 unique visits in those four days, so nearly a quarter of the visitors actually signed up.
The press was exciting, but didn’t really wallop us like we feared/hoped. We certainly have more participants as a result, but the project remains largely local and manageable in size.
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Here’s the source data if you want to reference/play with it.