Here's my re-creation of Grace Note's query.
The numbers, as of 2014-01-16, are:
Reputation Threshold # Users % of Avid Users Cumulative % of Avid Users
-------------------- ------- --------------- --------------------------
150 1095 51.7241 51.7241
200 793 37.4587 89.1828
500 132 6.2352 95.418
1000 51 2.4091 97.8271
2000 30 1.4171 99.2442
4000 16 0.7558 100
Status Update, 2014-02-16: There's a bit of progress over the past month. It takes a lot of votes to promote a user to the next threshold!
Reputation Threshold # Users % of Avid Users Cumulative % of Avid Users
-------------------- ------- --------------- --------------------------
150 1182 51.2798 51.2798
200 857 37.18 88.4599
500 158 6.8547 95.3145
1000 58 2.5163 97.8308
2000 26 1.128 98.9588
4000 24 1.0412 100
Status Update, 2014-03-23: It's no use! The percentages are barely budging. Although we have nearly doubled the number of ≥4k users since mid-Janurary, we're also gaining a lot of new avid users. The increased upvote activity has undoubtedly been beneficial, but I'm not convinced that this query is a useful metric of the vitality of the community.
Reputation Threshold # Users % of Avid Users Cumulative % of Avid Users
-------------------- ------- --------------- --------------------------
150 1299 51.50670000000 51.5067000
200 924 36.63760000000 88.1443000
500 167 6.62170000000 94.7661000
1000 75 2.97380000000 97.7399000
2000 27 1.07060000000 98.8105000
4000 30 1.18950000000 100.0000000
Status Update, 2014-06-22:
Reputation Threshold # Users % of Avid Users Cumulative % of Avid Users
-------------------- ------- --------------- --------------------------
150 1545 51.346 51.346
200 1109 36.8561 88.2021
500 188 6.2479 94.45
1000 93 3.0907 97.5407
2000 36 1.1964 98.7371
4000 38 1.2629 100
Here's one way to do something about the problem:
Welcome intermediate users by reviewing their answers or their questions
These queries finds posts from users with between 150 and 500 reputation points who have been seen recently. Those posts are likely to be at least somewhat valuable, since those users are known to have made positive contributions already. Please help bump up these members to 500!
There is an updated query available that now includes graduated privilege thresholds here