There are two things to keep in mind with intellectual property: the theory, and the reality.
The theory is that there is a 'right' and 'wrong' of trademark law. If Facebook has stopped using the 'Wall' trademark, in theory that means at a certain point they can no longer maintain the trademark since you must be use a trademark to keep it.
The reality is that intellectual property is a gray area. When you take intellectual property cases to court you can never be 100% sure who will win (other than the lawyers). Although something may seem very obvious you can't guarantee the win, especially if the other side has a lot of money to spend on lawyers and you (e.g., the new startups) do not. A great example is the i4i vs. Microsoft case. I know one of the principals. It took a LOT of money, government lobbying and a huge legal team including many of the best IP lawyers in the US (i.e., huge financial backing) for the little tiny startup i4i to beat Microsoft in court, over many years, on appeal after appeal right up to the Supreme Court, on what could be considered to be a very clean IP case.
So I wouldn't expect (smart) little startups to start using the term Wall, because Facebook will probably nail them to the wall with litigation.