We had a software upgrade last night and a now have a new feature. You will find that your PM box is searchable. Open you PM box and scroll down to find the search function.
There is an update to social groups as well - but we still have some bugs to work out. As soon as we have, you should be able to start a new thread in a Social group instead of having one long discussion.
But to be honest, the PM's hold so little material (50 messages is pretty small & fills up pretty quickly), a search function really isn't that necessary. It took about a minute or less to find a saved message without a search function. I can't imagine needing a search to find a saved message out of 50 or less.