To use an instant messaging program, you need an account with a provider of messaging services. Some services, such as Yahoo! Messenger and ICQ, use their own messaging protocols, which are not compatible with others; other messaging services, such as Gmail and Livejournal, support the free messaging protocol XMPP, also known as Jabber.
Merkuro is a Jabber client, and can therefore connect (theoretically) to all services which support Jabber. There are thousands of Jabber services in the Internet; the original, where XMPP was developed, is http://jabber.org/; a few others are listed at http://xmpp.org/services/.