Google Plus - approx 200 million users as of 2012-05-27
  
  I am just following up on research from Paul Allen 
  https://plus.google.com/117388252776312694644/posts/bGJPTALDkDe
  http://news.ycombinator.com/item?id=2747710
  
  I added more recent samples

  | Date:      |  07/09/11 |   09/21/11 |   09/25/11 |   10/02/11 |   11/11/11 |    02/16/12 |    05/27/12 |
  | Approx:    | 4,700,000 | 29,372,543 | 41,308,415 | 45,942,757 | 53,690,671 | 111,501,048 | 189,275,539 |
  |            |           |            |            |            |            |             |             |
  | Maring     |         9 |         50 |         65 |         72 |         91 |         210 |         406 |
  | Shireman   |        12 |         43 |         59 |         65 |         74 |         135 |         205 |
  | Toepfer    |        13 |         68 |         79 |         88 |        101 |         181 |         285 |
  | Fredenburg |         3 |         23 |         35 |         39 |         49 |         105 |         172 |
  | Louderback |         4 |         38 |         53 |         58 |         62 |         111 |         166 |
  | Wilcoxen   |         8 |         20 |         40 |         48 |         51 |         112 |         196 |
  | Lewter     |         6 |         23 |         30 |         33 |         42 |         103 |         209 |
  | McTigue    |         9 |         49 |         61 |         65 |         74 |         156 |         256 |