Projects will often provide their own abstraction. A default logger is bundled so things work out of the box, but you can easily configure the logging provider, as seen in ServiceStack logging and NServiceBus logging. For example:
LogManager.LogFactory = new Log4NetFactory(true);
There is also Common.Logging, which was designed to address the logging framework dependency issue. However, I have not seen it used extensively in major open source projects even though it's been around a while.