You can really tell a contextual advertising network is growing when their quality control personel begin to ban questionable publishers. Barry @ Search Engine Roundtable points to some forum threads which conjer up memories of when Google AdSense began to grow and cut out publishers.

Various threds are being started from publishers who have received this message from Yahoo Publisher Network :

We have terminated your Yahoo! Publisher Network beta account ACCOUNT ID for breach of the Terms and Conditions and/or Program Policies. Your breach includes but may not be limited to sending traffic from users located outside the United States, in violation of Section 11(l) of the Terms and Conditions. In accord with Section 6(a), we will not include such amounts in your payment. We have refunded amounts generated from the non-US users to our advertisers and will pay any remaining amount owed to you in accord with the Terms and Conditions.

Read more about YPN starting to terminated accounts at http://www.searchenginejournal.com/index.php?p=2864.