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I GM'd a sparse few times for whodunnit roleplay groups before a hiatus (it was via Facebook Messenger, and my players were between 15 to 45).

I think the approach depends on whether you know the players or not in terms of acquaintanceship. If it were a friend whose posts are turning sporadic, diminishing, then I flat out PM them and ask, "Hey, everything alright past few days (or weeks, depending on how long this has plagued the roleplay)?"

After a gentle cushioning, I address the elephant in the room. Usually they admit they A) lost interest, or B) are waist deep with "life". Just accept it and move on after telling them you'll manage if they allow you to off their characters really soon after one or two more posts.

If they're strangers, I aim to be [s]blunt and tough like a Mjolnïr[/s] straightforward. In most cases, they'll insist they "can do it" or worse, they "want to do it, really!" when nahhh bro u ain't.

In this case I take their word for it, say I'll account them to their insistance, and leave it at that. No ultimatums or threats. When they start lagging behind again, I badger them once more. This time, with a "I'm just doing what's in best interest for the roleplay so the others who are able to commit can still enjoy further developments". One more offence and it's the highway for them. My best I did.
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[b]tl;dr[/b]
Like cavity, if you leave it be, it'll fester. So you usually just extract it before it spreads. Or get fillings (recruit stand-ins). Either way, there will be pain. Which sucks.