It still comes as a shock to some community members that WordPress.com runs advertising on free hosted sites, unless you’ve purchased the annually renewable No Ads upgrade to remove them. This is stated in the Terms of Service that everyone agrees to when signing up for a website here.
Recently however, WordPress.com has made a much appreciated step in making site owners more aware of possible ads. Here’s what I saw earlier when previewing a post on one of my other sites:
Clicking on the “tell me more” link brings you to a page explaining why you might be seeing ads.
Again, advertising is only shown in a limited number of places. If you are logged into WordPress.com you won’t see ads on sites unless the site owner is part of the WordAds program. The most common way visitors will see ads is via search engine results on a single post.
So while WordPress.com may not be entirely ad free for logged out visitors, WordPress.com is helping site owners be more informed about their advertising choices (Ads, No Ads or WordAds).
Update: The above notification also appeared after clicking on a New Comment notification I received, so I do wonder if ads are going to become more prevalent than before.
WPcomMaven 
I also saw it this morning while working on my test site, left a note in the support forum.
Once in a while they get the changes right. Should help to reduce the confusion and shock when people discover the ads they agreed to on their blog.
Was a pleasant surprise to me. I hope it will be as pleasant of a surprise for newcomers as well, as I could see some backlash resulting from it.
Blogger seems to survive without casually placing ads on my blogs there. I felt invaded when l got that message from WordPress!!
Hi and thanks for your comment. I’m sorry you felt invaded, but it really is a “good thing” going on. The ads themselves on WordPress.com do not appear on all sites and at all times, unlike Blogger which uses adsense and they *do* appear on Google’s sites and in their mail service, search returns, etc.
According to nearly every speech that Matt Mullenweg (the founder of Automattic, which runs WordPress.com) has made, WordPress.com makes far more income from paid upgrades than from advertising.
Edited to add: “has made”.
Thanks for explaining this. I wondered…
Thank you for the opportunity to kvetch. I think there is a huge disconnect between the efforts of some bloggers to bring a little artistry into their sites, and the crass adverts that appear there courtesy of WP.
Yes, unfortunately it seems that mobile ads are ubiquitous and more of a problem content wise than the ones appearing on the web. They both can be jarring.
From time to time a community member will post in the forums about inappropriate ad content without realizing they need to take a screenshot right then in order for staff to see what they’re talking about because ads rotate and are not always the same. If they fail to take a screenshot when they see an inappropriate ad, the chance may not recur. WordPress.com has been pretty receptive in the past to fine-tuning ad content, but again unfortunately, only after a community member complains about it. That which has been seen cannot be unseen.
The bills need to be paid somehow, ads are one way of keeping to doors open and the servers powered up.
It’s the old TANSTAFL thing – There Ain’t No Such Thing As A Free Lunch
Ummmm …. WordPress seems to ne doing well…. Blogger does not “force” ads on you.
Again, if you look at both the wording in the message they’ve added and this article and my previous articles on advertising, WordPress.com only shows ads occasionally to some of your visitors; Not all visitors all the time (except for sites in the WordAds program.) However, the user message about ads appearing will continue to appear until you dismiss it. If you don’t want any ads to appear on your site, you can buy the annual no ads upgrade. It’s your choice.
Very true and with 30M+ sites on WordPress.com, that’s a lot of advertising space. But in just about every interview with MattM he mentions that most income is generated on WordPress.com by upgrades. I don’t recall him saying advertising being a big income earner.