we've already crossed 50,000 users on Steemit, and I expect to see near 75,000 users in two weeks.

Using an excel spreadsheet, I've compiled the daily total accounts data from steemd.com (collecting the data between 16:30 and 17:00 Pacific time every day) for the last 8 days and here's how the user growth charts (with "best fit linear line", correlation value, and projected value - dotted line).

Your 75,000 members by two weeks is very close to what the best fit linear growth line suggests. Two weeks would be the 22nd of August, which comes in at about 70,000 by the projection on my chart.