I'm a little late to this discussion, but here's my opinion.
I think OR-exchange shouldn't be used for job postings. There are lots of other places that are optimised for job postings, and have more sophisticated features such as expiry dates, filtering by location and industry, etc. Without these features, and mixed in with the regular questions, I suspect that the job postings will have the effect of spam on regular users.
Announcements are slightly more appropriate, especially since the answers could be used to indicate actual responses that may be interesting to the community, eg:
- For conference announcements, the answers could be used to indicate whether the users here are planning to attend, and their experience of past events.
- For notices about someone passing away the answers could be used to share condolences and positive experiences that the users had with that person.
- For changes in the OR world (new journals etc) the answers could be used to discuss and debate the merits.
I do think that the discussions prompted above would remain interesting even after the "event" has happened, and perhaps should not expire.
However, we also run the risk of them turning into spam. I suggest that we restrict the ability to make announcements. I'm not sure how customizable the site is, but there are plenty of features that are only available to different "levels" of users, such as editing posts, re-tagging, etc. We should make the ability to post announcements a power-user ability, and if the site allows the user promotions would happen automatically at a certain reputation threshold.