Remember back when co-authoring an RFC indicated you had reached the pinnacle of Internet Engineering? Nowadays, instead of running code and rough consensus, people just define an API to a closed-source engine.
Twitter. The best use case I've heard for Twitter is quick social merges. Say you're at a conference (e.g. pycon) with lots of people you don't know. You track "pycon", and you get all the tweets mentioning it, which keeps you clued in to what's happening in the rest of the con. On the downside, Twitter's admins can't keep it up. With an average uptime of 98.42% for the last three months of 2007, it might be up during your conference's keynote. Or not.
Facebook / Myspace. You post there simply because everyone else you know posts there. Until you meet new friends who use another site, so then you post there too. Getting your content out of the old site doesn't work, so you have to build from scratch. Again. Each Time.
MSN IM / Yahoo IM. Similar to Facebook / Myspace, we end up joining networks together on the client side because these commercial IM providers refuse to interoperate. Implying that we have to maintain different user identities across all the providers, and match accounts with people we meet.
Compare these services to the following:
DNS. Funded by domain name and IP space leases, registrars operate a robust attack-resistant network of root DNS servers. Local nameservers can come and go, however the overall system keeps working (sounds like the epitome of the Internet ;). With a distributed system on this scale, change happens at a glacial pace (DNSSEC and internationalization), but you have the choice of running your own DNS server or moving your domain to any of a wide selection of providers.
Email. Your ISP or provider of choice hosts your email, absorbing the cost as its an expected internet service. Most providers have conquered the spam problem for their users, resulting in usable email. Spam causes two big costs: the cpu cycles to filter email, and the increased uncertainty of successful email delivery. However, if service goes downhill, you can port your email address to a new provider if you own the domain name it uses.
Federated XMPP (or IRC). Competing with the other private IM services, XMPP allows you to run your own local server, which you can link to other public servers. Like email, if service goes downhill, hop to a new server.
By using Twitter, Facebook, or commercial IM, we lock ourselves into their system. While for small, niche, or new services, this makes sense, after a certain point, services become crucial and we need to demand open standards and interoperability (like porting phone numbers between phone carriers).
We need to recognize that point when a service has become internet valuable, to push for an open replacement, and to support those implementations. If we don't do this, we'll create yet another ecological disaster: a non-open, non-interoperable, locked Internet.