Providing advertisement content via an advertisement proxy server

1. A method comprising:

receiving, at an online system, an ad request to present an advertisement to a client computing device;

accessing, by the online system, a plurality of bid requests from a plurality of different advertisers, each bid request associated with a targeting criteria and an ad tag, the ad tags comprising one or more network addresses identifying one or more locations on one or more computer servers where advertisement content for the advertisement can be obtained;

determining, by the online system, a set of eligible bid requests from the plurality of bid requests by identifying bid requests having targeting criteria matched by one or more characteristics of a user associated with the client computing device;

selecting, by the online system, one of the bid requests from the set of eligible bid requests, the selected bid request associated with a selected ad tag, the selected ad tag identifying an advertiser network address associated with one of the advertisers;

retrieving, by the online system, the advertisement content associated with the selected ad tag from the advertiser network address;

storing, by the online system, a copy of the advertisement content at a proxy network address associated with the ad proxy server wherein storing the copy comprises removing a tracking pixel included in the copy of the advertisement content, the tracking pixel is configured to track web browsing activity associated with the client computing device;

generating, by the online system, a modified ad tag associated with the selected ad tag;

replacing, by the online system, the advertiser network address associated with the selected ad tag with the proxy network address;

responsive to the ad request, sending, from the online system, the modified ad tag to the client computing device;

receiving, at the proxy network address from the client computing device, a content request to obtain the advertisement content; and

responsive to the content request, sending the copy of the advertisement content to the client computing device.

2. The method of claim 1 , wherein replacing the advertiser network address with the proxy network address comprises:

appending a network address associated with the ad proxy server to a network address included in the ad tag.

3. The method of claim 1 , wherein the modified ad tag includes a portion of the selected ad tag.

4. The method of claim 1 , wherein a proxy network address specifies a content delivery network not associated with the advertiser.

5. The method of claim 1 , wherein the online system is also a social networking computer server.

6. A method comprising:

receiving, at an online system, an ad request to present an advertisement to a client computing device;

accessing, by the online system, a plurality of bid requests from a plurality of different advertisers, each bid request associated with a targeting criteria and an ad tag, the ad tags comprising one or more network addresses identifying one or more locations on one or more computer servers where advertisement content for the advertisement can be obtained;

determining, by the online system, a set of eligible bid requests from the plurality of bid requests by identifying bid requests having targeting criteria matched by one or more characteristics of a user associated with the client computing device;

selecting, by the online system, one of the bid requests from the set of eligible bid requests, the selected bid request associated with a selected ad tag, the selected ad tag identifying an advertiser network address associated with one of the advertisers;

retrieving, by the online system, the advertisement content associated with the selected ad tag from the advertiser network address;

sending, from the online system, a copy of the advertisement content to be stored at a proxy network address associated with the ad proxy server, wherein storing the copy comprises removing a tracking pixel included in the copy of the advertisement content, the tracking pixel is configured to track web browsing activity associated with the client computing device;

generating, by the online system, a modified ad tag based on the selected ad tag, the modified ad tag replacing the advertiser network address with the proxy network address; and

responsive to the ad request, sending, from the online system, the modified ad tag to the client computing device wherein the modified ad tag is configured to cause the client computing device to obtain the copy of the advertisement content from the proxy network address.

7. The method of claim 6 , wherein replacing the advertiser network address with the proxy network address comprises:

appending a network address associated with the ad proxy server to a network address included in the ad tag.

8. The method of claim 6 , wherein the modified ad tag includes a portion of the selected ad tag.

9. The method of claim 6 , wherein a proxy network address specifies a content delivery network not associated with the advertiser.

10. The method of claim 6 , wherein the online system is also a social networking computer server.

11. The method of claim 1 , wherein the modified ad tag does not identify a location associated with the advertiser.

12. The method of claim 6 , wherein the modified ad tag does not identify a location associated with the advertiser.