I cannot find the share button in the activity on the new version of the Connect site.
In the old version it is Gray even though I connected my faceebook and google accounts in myGarmin.
Up until last night I was posting to Facebook on my PC via Firefox because I wasn't completely in Modern, I could switch back and forth. Today it won't let me. The little box hasn't been working in Modern.
I decided to try my Mac laptop with Firefox and it posted.
You also might try a different browser. I used Microsoft Edge on the PC and it worked. I'll try Google Chrome next and see. I think it may be a Firefox issue.