joined:June 14, 2010
...they're otherwise not likely to be helpful...
For the sake of giving the OP a more accurate answer I'll stick my hand back in the fire here again at the risk of having to listen to the chorus of boos concerning W3C and validation.
If you use the Unicorn Version of W3C's validator, and your pages are error-free and valid, you'll get 3 or 4 backlinks from a PR 10 domain per page. It's how you set it up in the outbound link that's important. And it's a bit of work because each outbound link is going to be unique. Also, let the outflow go because you'll benefit from the backwash. In other words do not tag it as "nofollow".
Once they've been triggered once W3C's website sets them up as bookmarks on their server so if a search engine bot asks for them in the future they exist and will be served with a 200 code header response. Within the source code of the page there are no "nofollow" attributes.
Even if you eventually remove the links from the footer of each page of your websites they continue to live on as bookmarks as long as the code on that page continues to pass validation.
Over the years, and still (last time I checked), I see backlinks in WMT from W3C, unique ones for select pages. Whether or not there is absolute value in them -- that I cannot answer with absolute certainty but they have never caused any harm.
I have one site in particular that I had implemented that on several years ago and sometimes google displays that domain in a unique way in the SERPs that I haven't seen very often. It's sort of like a hybrid of organic and local search but they devote about 1/3 of the "above the fold" page space to that domain only -- enhansing it with a map and extra snippits of site info, even though there are competing sites that fall in underneath it that don't get that same attention. Again, I'm not going to say it's related to any one factor such as valid W3C pages but it obviously doesn't harm the domain either.
And finally, if the outbound links are on pages that are not validating -- don't bother unless maybe it's a site like Robert is saying -- some sort of a technical forum -- maybe to promote awareness.
The other ones you ask about, facebook, Twitter, etc... those I tag with "nofollow" and it doesn't harm because the search engines have grouped them together in SERPs.
As a general rule (for me) if outbound links are sitewide, or nearly so, I feel it's better to "nofollow" them. Anything else worth linking out to is worth allowing "the juice" to go down the rabbit hole.
That's my story and I'm sticking to it like a dog on a bone until the day someone from a major search engine such as google, Bing or Yahoo! or from the W3C come in here and tell me otherwise.