Is the W3 validation really important to SEO nowadays ? I'm unable to validate the Facebook like and Plusone button with this... I have seen some tricks that should solve, but I was wondering if such validation matters ??
If your HTML code is not valid you risking your site not to be indexed by search engines or geting a low SERP ranking. Facebook buttons are not a big deal, but try to keep your main layout of page valid, also be careful with closing tags.
if you are on wordpress , i would say get premium themes they have worked for me and dont get those free premium themes , they have links hidden in them
W3C validation does help in getting indexed by ensuring that your site's code is high-quality and without errors. If you're running into trouble with a specific add-on, like the buttons you mention, I wouldn't worry about it. It's not going to prevent you from getting a higher search ranking. If you can fix it easily, I'd do it, but if not, don't spend a whole lot of time on it.