Branding
by Jeff
Image meta text: Actually, 'RSS&M' is kinda catchy.
This comic is in reference to a Firefox extension called 'Adblock', which exactly as the name implies, blocks ads on websites. So, without Adblock, you will see all the ridiculous ads (except for our ads, they are great!) that attempt to get you to click on them with their catchy slogans.
The first two frames are not far off from reality, in fact I may have seen them before. Facebook and Twitter are widly known social networks. Google Reader is an RSS reader built on the google look and feel. The meta text references putting RSS and S&M together for RSS&M.
In the last frame, github is an online collaboration tool for developers to upload code to work with other developers on their projects. So, presumably, the comic is speculating that there will be a new GitHub, this one for only lesbians. This is the pop-up windows attempt to attract clicks and in the comics case, it might have worked.
This xkcd is one of the rare xkcd's to use color.

August 19th, 2009
First!
December 30th, 2010
Most likely the real ad that the cartoon is parodying is the one that promotes a site as “the Napster of porn”. I haven’t seen it lately, but it used to pop up pretty often.
December 29th, 2011
I’ve definitely seen the Facebook for adults/porn/sex ad before, so that one is legit.
Just to point out the entire punchline of the joke, the strip starts by advertising the “Facebook of sex!”, which suggests that the site will make it fast and easy to find lots of people to sleep with. The comparissons become increasingly remote as the strip progresses to the point where 99% of people reading the ad wouldn’t know what github is. Thus it’s not eye-catching and doesn’t connote anything to the average reader. Similarly, while people mightk now google reader, it’s not really a popular trendy “cool” thing that would be good to associate with sex/porn.