Piwik should parse (using referer information, referer domains are already shown) advertising information.
For example Google pageads referer urls are quite simply formatted (unnecessary information stripped out, original url is really long):
http://pagead2.googlesyndication.com/pagead/ads?client=ca-pub-...&dt=...&lmt=...&output=html&url=[source url]&ea=0&ref=[previous referer (where user did come to source url)]&kw_type=radlink&hl=fi&kw0=keyword1&kw1=keyword2&kw2=keyword3&kw3=keyword4&okw=[primary keyword (?)]
Some statistics may be nice. How many people did come from from advertising sources, what keywords were used and so on. Of course google gives same statistics but google is not the only advertising source. And it may be nice to cross-check google statistics accuracy.