New and Improved; Made-Up Definitions:
Altruistic Vampire -one who subsists on the dopamine released when they help another; for whatever personal motivation: an addict of altruism; someone addicted to answering questions online. -Similar and often found in conjunction, though not to be confused with: Stack Exchange; Rep-Whore.
Cyber Vampire -one who subsists on the dopamine released when they telecommunicate with other people; someone addicted to interpersonal relations which can only be found online.
Until I looked it up, this is what I thought Help Vampire meant (apparently it means ne'er-do-well).
Any 'new' word is just window dressing for the word addict. Like this 5yo question that seems to be getting along just fine: How addicted to Stack Overflow are you?.
Internet traffic tracking sites have a category for addicts. IIRC, of the two SE sites that are on the top 100 list, ~50% of its users are addicts. I hope I can safely make the assumption that way more of that 50% are altruistic than help. Therefor I just call myself them (for good or evil) a
Stack Exchange Addict.
For a definitive answer we'd need to know why they're addicted; are they a rep whore or one of the two vampires? We need more information to narrow down this obsession. However once we have it, we no longer need this mysterious all encompassing word for "someone who is addicted to a Q&A website".
Although fitting, I believe it would be a stretch to call them trolls. It would be immediately misunderstood; crossing this bridge requires you to read my answer to your question.
We're all trolls. It just depends on which side of the Schwartz you're on; the Up side or the Down side.
Best Answer
A recluse is a much better word if you want to say that they avoid social contact, without passing any judgement on their behaviour.