Online ad company ValueClick has warned investors that earnings in the second half of year would not meet expectations due to weakness in display advertising, paidContent reported Friday.

After Yahoo's poor Q2 earnings report, investment bank Cowen also cut its expectations for the online ad industry's growth. According to the analyst's report, its estimate for U.S. online ad market growth has been lowered to 16 percent year-over-year, compared to its previous projection of 19 percent, paidContent reported Friday.

Cowen's downward revision follows Magna's pessimistic adjustment for online ad spending forecast earlier this month, which it lowered to 12 percent growth for online ad revenues this year, down from the original 16.5 percent in December.

In addition, TNS in June said display growth was already dropping, with Q1 ad dollars for that segment plummeting from last year. Lehman Brothers also lowered its expectations for internet ad spending, paidContent reported.

However, a few analysts still remained optimistic about U.S. online ads, including PricewaterhouseCoopers and ZenithOptimedia. For example, ZenithOptimedia expects global internet ad spend to grow 26.7 percent this year, according topPaidContent.

While display has been impacted by the economic slowdown, Cowen forecasted that the U.S. search market will still grow 22 percent, much like the previous prediction. Google will still make up 45 percent of the incremental growth in the online ad market this year, versus 12 percent for Yahoo and four percent for Microsoft, according to estimates.

Moreover, Cowen still held a strong belief that Google will be able to monetise YouTube, Google Apps, and non-search related ad serving. The rating for Yahoo was neutral, since the company kept on losing share in search, while its display ad business was more sensitive due to the worsening economy, paidContent reported.