As noted, Trulia can delete answers it considers spam. Other users can report answers that appear to be prohibited, spam, duplicative, etc. Or the original poster can remove the answer.
As Dunes notes, sometimes promoters will mine old answers, going back years, so they can post their answer--whether it's how to start a foreclosure cleanout service or how to do a short sale, or a dozen other things. Those answers--especially when they consist primarily of a link to a web site that sells something, aren't particularly helpful.
For example someone creates a profile, then cuts and pastes the same answer 20-30 times to every clean-up question they can find (even 2-3 year old ones). If Trulia sees that it is considered Spam and they will delete all of the answers....
For what's allowed and not allowed you can read the Community Guidelines..http://www.trulia.com/guidelines/
Another example....At the end of your question you put.."For more information about our service, please see http://XXXXXXXXXXX"....According to the Guidelines that could cause Trulia to delete your question as Spam!
On a blog I can, not on a Q&A question.
I had a similar problem. I asked trulia staff about it. My post was brought back.
Ask Emily, she should be able to help with this. I sometimes wonder if trulia has bugs that eat messages once in awhile.