Some brands buy textads, others take over every Xanga member page in existence - like Sprite is currently doing with their "Sublymonal" campaign.
Not all Xanga members are happy about this, Reality_1 complains "It pisses me off just a little that they can do that without my consent." but he also recognises that if they really wanted complete control over their webpage they could have opted to not use a "free" (read advertising supported) service.
So here's what the subliminal stuff looks like in action, using Reality_1 page as an example.
Without touching the top right lemony thing, it's a grey page with a duck-logo (see image above)
But touch the lemony thing in the upper right, and presto - it is now sublymonal all over! All blinky-gif and ugly as hell. In the upper left, the Xanga.com logo has been changed to YOU LOVE LYMON. Subtle. ;)
You should check out the caffeinegoddess' post about the Uncola wars if you missed it - and here are the Sprite ads from this campaign.