The real difference I see between google and twitter is space and focus. Google had room on their clean results pages to place ads, and they also had users that were used to skimming and overlooking a lot of the text on the page anyway. Ads were not a huge problem for google. Twitter's app users don't have the space on their feeds for a few square centimeters of ad space, especially because we don't skim and skip over tweets (unless they look like the have lots of @s and #s). Tweets are like prepackaged feelings already. Someone who barely read tweets would have nothing left to intake. Twitter needs to find a way to place ads without taking up space or annoying reading time. That, or find a place on the page which doesn't LOOK like its taking up space. Meaning, if the bottom cm is always an ad, it isn't dropping down into my tweets. I just have a slightly smaller screen, which I will acclimate to quickly and won't annoy me regularly. Also, the ads can be more expensive that way. Put the ads on the screen all the time, not just some of the time, and they won't impede upon my twitter space, they'll sit within twitter's ad space.