This may sound simplistic, but I think this happens due to the intent of the user when using some service.
Simply people with commercial intents, willing to spend money on something, are more likely to use a search engine than a social network.
Simply people with commercial intents, willing to spend money on something, are more likely to use a search engine than a social network.
Provided there are incentives for people in social networks to list the commodities they own, it would be rather nice, when you're about to buy a commodity, to get (on-demand) feedback from friends that already have this type of commodity (as opposed to the feedback from the whole community). Example: how to choose among the 5 different types of GPS'es? If a friend tells you that model A works flawlessly, you might just stop cross-analysing all available models on the market and just buy model A (via a sponsored link, next to the area where you exchange feedback in your social network).
The whole web 2.0 trend with user-generated content is nice, but it's limited because one does not trust anyone equally. You need personalization (a trend in web 3.0) and/or small communities (where people can relate to each other easily, and not "to the whole world"). Hacker News is an example of the latter, where I'd say the users are much more homogeneous than those on Reddit (and that's probably the main reason why people migrated from Reddit to Hacker News).
All in all, I do see potential for social ads, but in an improved form of what we have today.