Members as of now, 11:19 on 27th December, 2011: 148,895
Only 1,105 mebers to go!
The Terminator and Sweetpea guessed the 26th December, and so have lost.
Here's a (very rough) graph of the numbers so far. Note that the graph points don't take account of the time of day when I made a note of the numbers. However, I'm feeling optimistic about my guess!