Whenever a website (or anything offline that might put rankings or whatever online) wants me to use my real name, I usually default to "Alex", "Alex M/MJ" or "Alex (gibberish)" depending on their guidelines. What they view as a policy that "encourages openness", I view as something that will guide first impressions when people google me unto eternity.
Even if it's not controversial, it's the first thing a potential client or employer sees when they type things into google, and I don't want to be reckless with that. I don't want to have to view every minor action that I make on a website as something that's potentially going to be on the front page of google's results for my name.
When I was in HS, my friends had a poker league, and we all had nicknames and personas - someone put up a page with hilarious (but decidedly unprofessional) caricatures and quotes and it was the first thing that showed up under my name. I noticed it and asked my friend to change it, which he though was hilarious but understood and did.
Same for Meetup. A long time ago I went to some meetups that could potentially be alienating to some folks (agnostics and whatever, way back in the day when I felt it was worth arguing about) and BOOM, google front page for my name. If I had a client who was somewhat religious, I don't care at all but I don't want that to show up and all of a sudden I'm accidentally alienating people. For showing up to something.
Whenever I run a 5k or do anything remotely prestigious I always use my full name so that the front page of google just gets filled with that kind of thing in case anything else that's potentially a liability pops up and I don't notice it. It's annoying.