Just reporting a couple of points... try typing this into the activity stream as a story update:
Link Tests... http://www.mysite.com. Testing http://www.mysite.com.
2 issues to report with what gets displayed (and note this is if the person doesn't use the 'add link' dialogue that pops up and just closes it down which my users tend to do if they don't like the extra detail it's providing)
1. The www. doesn't get picked up unless it has the http:// in front of it. Most people wouldn't add that.
2. If you do add the http:// in front (my second example in the code above) but then finish the message you're typing with a full stop (as good grammar people would!) then it gets included in the hyperlink and so that ends up failing.
So solutions (hopefully not too tricky), scan for any www. and convert that to a link and ensure that if the text ends in a . then that is removed from the hyperlink.