The link to the page is here. (Hat tip: Stand Firm) You need to scroll down, enter a diocese (and a parish if desired), and it will give you a pdf graph of members, worship attendance and plate/pledge contributions.
The Diocese of South Dakota seems to have plateaued at around 2,100 in average Sunday worship attendance (ASA), down from 2,600 ASA in the year 2000. Financial giving was up after two years of decline, but still not exceeding the amount given in 2003. Membership (always a tricky and unreliable number) is slightly up after years of decline. Some of the declines might have been due to better reporting (clearing inactive people off of church lists), so it is hard to track a trend there.
Flagship liberal congregation Calvary, Sioux Falls continued to decline in all categories. This is a church where Bible-bashing Bishop Spong is taught as "Christian Education" and a homosexual activist group holds its own services and leads classes to enlighten the rest of the congregation.
Emmanuel, Rapid City, the largest West River parish, showed a noticeable dip in ASA, although it claims an increase in members (go figure). Their financial giving is up, but not back up to the level of year 2003 (and their ASA has fallen each year since then). Please note that the earlier version of this post included incorrect information. Emmanuel is NOT home to an Integrity Chapter, although a local advocate of the Integrity agenda has identified as an Emmanuel member from time to time.
Moderate parish Trinity, Pierre showed a marked gain in ASA and in financial support, although still below the giving level of year 2002.
Traditional parish Good Shepherd, Sioux Falls had a solid ASA increase and reached a parish best for financial support.
The ASA increases at Trinity and Good Shepherd might explain the apparent "levelling" of diocesan ASA for 2006. Statistics in small dioceses can be difficult to interpret as even a small change in a small parish can create a major blip for the diocese.
But the overall weakness and irrelevance of the Episcopal Church here is suggested by the fact that Lincoln County (includes the south side of Sioux Falls) population grew by 45.9% in the years 2000-06, reaching just over 35,000 residents according to the US Census Bureau. There is no TEC church in the county, and no effort to plant one. The combined ASA of the three TEC churches in Sioux Falls (Calvary Cathedral, Good Shepherd and Holy Apostles') hovers around 300. Minnehaha County, in which they sit, has a population of over 163,000, up 10.1% from year 2000.