Alex... I can always count on you and Elearner to ask these really great questions!!
After reading the link that translationsnmru posted, I thought about how I would would both would write and say my responses. And I think I would interchange them depending upon the reason for them. Example:
Your first one was about the forum in general:
"On seeing the phrase in one of the posts here on this forum" And I believe I would say and write it that way. IN a post ON the forum.
However, I would actually say this as "I am new TO this forum"