If you don't see a "log in with Facebook or Twitter" button on a website where you expect to see one, check AdBlock's options to see which filter lists you're subscribed to. (To do that, click the AdBlock button in the browser toolbar and select Options, then click the FILTER LISTS tab.) You're almost certainly subscribed to either the Antisocial or Fanboy's Annoyances filter list (or both). Those lists block social media widgets that, among other annoying things, can track your browsing behavior.
If you prefer to keep those filter lists enabled, just click the AdBlock button in the browser toolbar, select Pause AdBlock, and reload the page. Log in using your social media account, then unpause AdBlock the same way.