Au pair couldn't talk through the the phone/skype with her parents that were on the other side of the globe after her working hours.
Of course APs can talk with their family and friends after their working hours (I don't think anyone minds 3 am calls due to an emergency - I am talking about the usual chatting about your day with your mom here). They do, however, still need to get enough sleep to be able to fully function during said working hours.
From DC, Europe is between 5 (UK) and 7 (Finland etc.) hours ahead. An AP can work 10 hrs per day, so let's assume AP has a toddler to watch, she might possibly work between 7 am and 5 pm. At 5 pm (EST) it would be between 10 pm and 12 am in her home country for an AP - possibly early enough to skype with her (working) parents, most likely early enough to skype with friends. When she gets up at 6 am, it would be between 11 am and 1 pm in her home country, a quick chat with parents or friends (depending on their working hours) would likely be possible. Most toddlers still nap during the day which would also be an option to talk with friends or family at home.
The time difference between EST and let's say Brazil is three (!) hours, there is no need to skype with your family at 3 am. The time difference between EST and Thailand is 12 hrs, while a 3 am EST call is probably extremely convenient for the family in Thailand, AP could just as well contact them before starting to work (6 am / 6 pm) or after work (7 pm / 7 am). It's about making choices and making good choices. We all know that when we show up for work we need to be awake and alert to be able to do our job. The same is true for APs. APs who work a split schedule should be able to find the opportunity to talk to family and friends during their time off.
If family and friends can't talk from x am to y pm because they need to work (or sleep) then so does the AP. AP's family can't contact AP at 3 am their time because they need to work the following day? Okay, well, same is true for AP. They need to work something out that suits both sides. There is also no need to constantly be in touch with your family and friends when you are an adult. I don't skype with my mother when I am in the office, my kids' teachers don't call their best friend in the middle of a lesson, a Walmart cashier can't walk away from a register to message her husband while on the clock - why should an AP be held to different standards than (nearly) every other working adult.
Yes, young adults can function on less sleep. Heck, when I was 19/20 I could spend all night partying (or studying) and then fully function for a full day at college. 6 hours of sleep were plenty. However, it was 6 hrs of uninterupted sleep. And if PP says "We had an AP who was up all night talking with friends back home" it really doesn't sound as if their AP got anywhere near that amount of downtime. Being up "all night" talking with friends at home if you are working during the day simply isn't the greatest idea for anybody. There is weekends for that.
It's the same with texting. There is also a huge difference between sending your mom three texts during the day ("Morning mom, I am awake", "Look mom, the funny thing Larla did today" and "Hey mom, how much flour do you use in your pancake recipe?") and constantly being on your phone, texting you mom, your dad, your third cousin, that girl you met in kindergarten plus your new best AP friend next door.
The problem is, as soon as it becomes an issue (AP neglecting her work over texting/skyping/...) you run into needing stricter rules than you are happy with yourself. It's like jobcreep. It's much easier to say "you can't do x" than saying "you can't do x, unless a or b or c" or "you can only do x a certain number of times." We have a strict "no phones at the dinner table" rule for everybody. There is a small shelf in the hallway, outside the kitchen, that ALL phones go on before we start eating dinner, everything can wait 30 minutes. AP is a multi hour flight away from her family, there is nothing she can do anyways (same is true for us, there is nothing we can do other than comfort whoever is calling).
Infant, baby or toddler will be waking you and your Au pair every night couple of times. Please don't even mention here about getting enough sleep in that kind of house.
As an adult (no matter if you are 18 or 26) you should be able to wear headphones when skyping with your family/friends and to keep your own volume down enough as to not wake everybody. You are not a 3-months old infant who can't help it or a 2 year old who woke up from a nightmare crying. There really shouldn't be any reason to wake the rest of the family if you have your door closed and use your inside voice.
Thinking back, I often called my mom when going to bed, because that was when she was getting up (midnight CST, 6 hr time difference). As the toddler I was caring for napped around noon that's when I talked with friends. There are also, conveniently, days off. I also don't think I ever woke anyone up when I was talking to family. With a mindset of "hey, baby woke me at 1 am and 3 am so I can blast music at 5 am / yell at my boyfriend at home at 5.30 am" people should simply not become APs. That's not how mutual respect works. Baby can't help it, you f-ing can.
The toddler I cared for thankfully usually slept through nights. If she didn't, her parents got her within minutes. The baby that was born right before I left coslept with HM/HD for the first months so that they could tend to him easily at night. Not having to get up to care for them meant hearing noise and going back to sleep within seconds. The only thing I learned is impossible with children is sleeping in because the kids will be up no matter the day and school-aged kids don't care that their AP was back home at 3 am and wants to sleep later than 8 am. Good thing to learn that at 19 and plan accordingly.