I've contributed to the condolence threads about a handful of times.
Twice because I knew the blokes personally. The other couple was earlier in the year, when casualties were being reported daily and it got to me a bit.
Other than that, I leave them alone - I agree there is not much need for one-word "RIP" responses. If you have a genuine dit about the deceased, or even advice for the family concerned (contact groups, etc), then fine.
I'm off to the Legion on Saturday all being well, i'll contribute then.