Re: URL Shortening
My favourite url shortener name so far I think is ‘Beam.to’
We push blog posts out to twitter via Hootsuite, which has its own url shortening ow.ly (see what they did there?) So if you’re looking to do that kind of thing, a separate twitter / sn client might be the way to go.
If you want to push members blog posts out to twitter, then any compatible WP plugin should work, you would just need to install a sitewide WordPress > Twitter feed plugin so that members can use it on their own blogs too.
We use shortening services because it’s the nature of the beast but apart from the worries @stwc mentions above, as a few people point out, the danger of course is that if that url shortening service should cease to be so will all your links! Just a thought…