I've dropped Twitter. Their iOS client regularly pushed so many ads down my throat (in stream) that (a) I noticed and (b) I noticed it every day.

When I got to the point of blocking every advertiser (confirmed views +1!) but they popped back up under different special handles I resigned myself to defeat.

I will miss the serendipitous discovery of tidbits of information that it enabled, but the signal/noise (or rather, content/ads) ratio was overpowering. It needs to make money, but I think it's killing itself or at least transforming itself into the equivalent of TV -- content producers broadcasting to a largely passive audience.