You've probably already heard that SL's annual Relay for Life raised $375,000 in donations that'll go to the American Cancer Society this year (that's 75.5 million in Linden Dollars, before being converted to cash), but here's a figure that's just an interesting to me: According to Poppy Zabelin, who handles SLRFL's publicity, this amount came from approximately 115,000 donations.
That's an impressive number: assuming some of the donors made multiple contributions, I'd estimate SLRFL had somewhere in the range of 40,000-100,000 individual donors. While Second Life has about a million monthly unique users, only about 464,000 of them are economic participants who received or distributed L$ last quarter. So if my 40-100K estimate is right, 8% to 21% of SL's economic participants contributed to SL Relay for Life, which is easily the most prominent non-profit fundraising effort in Second Life. So I'd say that's the maximum number of participants you can currently expect to get involved in a given in-world fundraiser.
This is, by the way, quite a leap in donations from SLRFL in recent years; in 2010, the total was US$222K, in 2009, US$275K. While SL itself is still at a growth plateau, it's good to see the existing userbase putting more of their money into a worthy cause.
Image from Daniel Voyager's blog.