Why Ohhh why don't they build their own contextual ad system. Self service to buy ads and there shown on a users stream, search results, rss feeds and api calls.
Will it cost them users? I doubt it. Who refuses to use a service simply because they show targeted ads to users. True there will be a few who leave or choose to block ads, but it doesn't matter because they aren't making any money from them anyway.
Twitter have the numbers. It's time they done something with them.