It’s like with every other discipline. You can call yourself specialist, professional, consultant or whatever but you can’t refer to yourself as an expert unless others do, or rather only other people can call you an expert. Otherwise you’re simply boasting. So true social media experts won’t tell you that they are experts actually. They will point of what they accomplished, how long they are using specific social media “channels” etc. They might cite others referring to them as experts in reviews or testimonials.

Imagine someone calling himself media expert. What media is s/he an expert in? Print media? Radio? TV? “Multimedia”? There are journalists, photographers or scientists studying media but there are no sicla media experts because there are no unified social media.