Email (well, the majority of it anyway) is one nice standardised SMTP network -- any mail server can send to any address, finding each other by the domain. To some degree client protocols are also standardised (IMAP or POP), and although there are exceptions (e.g. Exchange protocol) the servers and clients still generally support the standard protocols as well.
Instant messaging, presence, and related networks are, however, still fragmented.
This means that you need a Lync client (Office Communicator) to talk to a Lync server, a Skype client to talk to Skype (for IM and presence), Windows Live (MSN) Messenger to talk to MSN, etc.
It also means that networks are not fully connected to each other. Slowly, very slowly, there has been some convergence, e.g. Windows Live and Yahoo! have been interconnected for a while, and recently Windows Live connected to Facebook. Office Communicator/Lync does have a Public IM connector for some networks, but it requires additional configuration and/or licensing (cost).
There are a few open/standardised protocols, e.g. XMPP (formerly Jabber, developed in 1999, standardised in 2004) and SIMPLE (part of the SIP VoIP protocol family, also developed in the late 1990's, standardised in 2002), and they do have DNS discovery protocols, but servers often still block connection unless specifically configured and/or additional connectors are purchased.
e.g. You can look up sip:email@example.com via "nslookup -q=srv _sip._tls.readify.net", but can't connect unless you are a federated partner, or you can look up "nslookup -q=srv _xmpp-server._tcp.gmail.com" or “nslookup -q=svr _xmpp-server._tcp.chat.facebook.com” (most XMPP networks are open).
It is very annoying to have separate applications running locally for Live Messenger, Skype, and Office Communicator -- so annoying that I often don't bother to log in to them and simply shut them all down.
In contrast, I have one email client and it 'just works'.
(Similarly with the telephone network, I can call (almost) any number in the world and it just works.)
Part of the problem is the co-opting of domains by services like Live Messenger. I always thought it was silly that you could create an account "firstname.lastname@example.org" on the Windows Live/MSN system, because there isn't really anyway for DNS lookups to work; it looks like an email address, but really the "example.org" in the address has _nothing_ to do with the real example.org domain (it’s really ‘email@example.com’@msn.com).
Eventually once interop started this problem became a hurdle requiring work arounds. For example with the Lync/Office Communicator public IM you specify the address in a particular format: "alice(example.org)@msn.com" (if you aren't one of the real domains such as hotmail, msn, passport, live, etc), but this are still point-to-point integration solutions and not general solutions.
Facebook recently opened up its chat network with an XMPP interface, plus the new Lync Server 2010 has a free XMPP Gateway component (unlike federation with other networks, like Yahoo!, that require additional licensing).
There is slow convergence (but very slow), but even once the IM and presence hurdles are overcome there is probably still a way to go for voice and video chat. One day maybe you will be able to use your iPhone Facetime to video call your corporate Lync server, or Live Messenger to video call Skype, one day...