check_http_curl.sh

Save plugin in $USER1$ dir, in my case (Ubuntu 12.04) it is:
/usr/lib/nagios/plugins
and make it executable.

Command definition:

/etc/nagios3/commands.cfg:
...
# Check HTTP
define command{
command_name check_http_curl
command_line $USER1$/check_http_curl.sh $HOSTNAME$ $HOSTNOTES$
}

Hosts definition:
$HOSTNOTES$ is the string that the plugin will search in the web page source.

/etc/nagios3/conf.d/generic-host_nagios2.cfg
...
define host{
host_name www.site1.org
checks_enabled 0
use generic-host
notes https://www.facebook.com/site1
}

define host{
host_name www.site2.org
checks_enabled 0
use generic-host
notes info@site2.org
}

Group definition:

/etc/nagios3/conf.d/hostgroups_nagios2.cfg
...
# Web Sites
define hostgroup {
hostgroup_name web-sites
alias Web Sites
members www.site1.org,www.site2.org
}

Service definition:

/etc/nagios3/conf.d/services_nagios2.cfg
...
define service {
hostgroup_name web-sites
service_description WWW
check_command check_http_curl
use generic-service
notes Check if site is really down.
}