How about adding loggly related configuration in App.config file like below.
I am using it for hosting WCF service within Window Service and it works fine for me.
<configSections>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
</configSections>
<log4net>
<root>
<level value="ALL" />
<appender-ref ref="LogglyAppender" />
</root>
<appender name="LogglyAppender" type="log4net.loggly.LogglyAppender, log4net-loggly">
<rootUrl value="https://logs-01.loggly.com/" />
<inputKey value="abcdefgh" />
<tag value="log4net" />
</appender>
</log4net>