The list of compatible numbers is as follows:
119, 121, 129, 141, 159, 161, 169, 171, 189, 201, 209, 219, 221, 231, 249, 259, 261, 279, 289, 291, 299, 301, 309, 319, 321, 329, 339, 341, 351, 361, 369, 371, 381, 391, 399, 429, 441, 451, 459, 469, 471, 481, 489, 501, 519, 529, 531, 539, 549, 551, 559, 561, 579, 581, 589, 591, 609, 621, 629, 639, 649, 651, 669, 671, 679, 681, 689, 699, 721, 729, 731, 741, 749, 759, 771, 779, 781, 789, 791, 799, 801, 819, 831, 841, 849, 851, 861, 869, 871, 879, 889, 891, 899, 901, 909, 921, 931, 939, 949, 951, 959, 961, 969, 979, 981, 989, 999
Of those, I'm going to guess it ends with 69 (leaving 169, 369, 469, 669, 869, 969) based on society these days. My original guess was 69 but that's not a 3-digit number.
I'm also going to assume that it's under 200 because you likely have a basic idea of how many posts most threads get on the forums. That leaves post #169 to be the most likely number in my opinion. I could be very wrong but that's my best guess.
EDIT: Based on the above, I technically did think of the number, but in a "here's your number in a sea of every other possible number" sense lol.