Rosalind - 10:57 am on Feb 13, 2012 (gmt 0)
I wouldn't say it's doomed, but you'll be missing out on quite a few side benefits: pictures of your face in the SERPs, better name recognition which might lead to more click-throughs to your site which could be factored into the algo. And social shares and likes certainly are used as signals with personalisation on, so they matter.
Balance that against the time you spend developing your site. People will still Tweet your stuff if it's good and you let them know it's there, and you don't need to have "like" buttons for them to like it.
One of the difficulties is we're at a transitional phase: Facebook has probably reached its peak, but even with SERPs integration it's no sure thing that Google+ will take its place. It could be Pinterest, Foursquare, LinkedIn, Twitter, or any number of others instead. So by choosing not to take part in that you're not wasting any effort on social sites that are either on the way out or are not getting much use yet.