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.
Continue reading [Rant] Why are IM/presence networks still fragmented?(3 min read)