@Sgt_Kickaxe - perhaps not in the far future SERPs, but certainly for the next 36 months, yes ... but by then 'poplet' integration in the browser bar will be far more common and utilised by visitors, meaning the website itself will not need to carry the extra code - Bing is already putting the facebook like button in as standard, will they carry 'twitter' and '+1' for their rivals, and will Chrome do the same for facebook? Only time will tell.
However, I think vastly popular sites with content only available to 'subscribers' and the increasing number of sites moving over to subscription based models (who block search engines from crawling their inner content) will be fine ... here word of mouth through 'social' will help increase membership, however it is the 'content quality' which will be the main draw to these 'walled gardens', the SERP's not so much.
So in reverse to your question ... is the biggest problem for search engines the acceptance (however reluctantly) of an internet which is no longer 'free' for the best
At the moment advertiser only based models of payment cannot pay for the best writers, and turn a decent profit, and UGC can only compete for so long if professional writers and skilled content creators start hiding away their content - i.e. the New York Times blocks content sharing from the HuffPo etc. and they take copyright infringements more seriously. If subscription based models take off then the best content on the internet will get 'removed' from the SERPs, and perhaps only 'shared' and found through 'social'? Therefore 'social' could be the death of SERPs?
[edited by: JoePublisher at 11:54 am (utc) on Jun 2, 2011]