As far as the work thing goes, I think you may be being a bit soft- where I work, if any of us call in sick it is hugely inconvenient for the rest (means someone else has to go in on a day when they wouldn't normally, find childcare at short notice etc, or work a 12hr+ shift) so unless any of us is so ill we can't stand, we would never take the day off, so I find it really surprising that your coleagues can take a day off because they got up in the night a few times!!
having said that, I think some people cope better without sleep than others. I am a vet, so was used to getting up in the middle of the night, getting dressed, getting in the car and driving in all weather to some obscure farm, then spending maybe 2 hours struggling to calve a cow (eg), get home at 5am, knackered and filthy, have a shower... then get called out to something else, and still have a full days work ahead of me, so getting up a few times in the night to see to a baby wasn't such a huge deal to me, as it was to friends more accustomed to a full night's sleep! I am also a person who naturally doesn't need a lot of sleep (am fine with 4 or 5 hours), so I do sometimes find it hard to empathise with those who really struggle without their 8hrs. Still not sure it warrants a day off work, though!
I have 3 under 5, but remeber how hard it was when I just had one! It took me so long to get her into a good sleep routine (used to sit holding her hand for 4-5hrs a night, crying with helplessness ) Somehow the next two were a whole lot easier, and we haven't had sleep problems at all with them, so I'm not sure that the numbers thing actually works!!
As far as your friends go, I can see why you would be a bit annoyed- you sound like the kind of person who, having made an agreement to do something, will stick to that no matter what (I can be a bit like this- HATE letting people down), but I suppose not eveyone feels like this- they probably think that as a friend with children you will be sympathetic, I don't know. I think, though, that maybe they would be better not to make arrangements until their circumstances will allow them to be more confident that they can honour them. I missed a friend's party last night, but I told her when she invited me that I would make it if I could, but there was a strong possibility I wouldn't be there, due to ds's chest infection, so hopefully she won't be too annoyed. I think if I was you I would either make arrangements with a whole group of people, so that if one or two cancel I could still go out with the rest, or just arrange to go out with your dh and see them there IF they can make it- less pressure, and less for you to get annoyed about! (although where you get the energy for a busy social life with 3 kids AND pregnant with twins amazes me!!)