ClickZ lists the big adspenders in online advertising for last month, citing numbers provided by TNS Media Intelligence. The top five advertisers in April were Tickle by Emode ($23.8MM), Vonage ($21.8MM), South Beach Diet ($18.6MM), LowerMyBills.com ($14.1MM) and HP VAR Computers ($13.9MM). A little bit of number crunching reveals that among the top 50, the tech sector spent the most on online ads ($62.1MM, with eight top 50 players); financial services accounted for $56.2MM, spent by 15 advertisers in the category; followed by online dating ($27.1MM), with two entrants in the list.

The methodology behind the data doesn't take into account the vast amounts of barter impressions, affiliate marketing impressions, and other mechanisms through which many ads make it online without a great deal of money changing hands.

In fact, the top 50 list appears to be a collection of some of the most active barterers and affiliates on the web, perhaps indicating these ranks are much more representative of share of voice than of spending levels.