With a SoloLearn PRO subscription, you will have an ad-free experience with access to a more customized experience that includes setting learning goals and access to personalized activity insights. On the social front, you can see who's viewed your profile and who's learning nearby.
Also, you get a cool tag, and you get no ads (I admit, sometimes they are strange).
You can see who looked at your profile and people who are near you, though it is not that important. You can also see your statistics on how long you spent time on challenges, Q&A, and courses, though this is also not that important. Part of the new sololearn courses used to Sololearn Pro only but I think that was changed.