It's on case by case basis, but I would say that they are trying to build a user base first and monetize later on. Perhaps they have Venture Capital (VC) funding that is keeping them going for the moment.
A prime example of this is Instagram, never made a penny, sold for $1bn, they effectively sold their user base to Facebook.
Tumblr is another great ...
Google's updates to its search engine algorithms nullified the impact of having domains that exactly match keywords, more commonly referred to as Exact Match Domain, abbreviated EMD.
Some argue there are still benefits to having keywords in a domain name if your site also meets Positive Engagement Metrics. But the purpose of the updates regarding EMD were ...
Maybe not yet, I remember when Youtube and Facebook put no ads on their pages and video (pleade CMIIW but at least in my country was).
But there are many ways for startup to pay the bill: donation, pro product, member or page promotion (e.g in your Twitter suggestion, page suggestion).
Money is not always part of the main purpose.
My friend told me that a ...
I do not think there is any significant problem with having your contact info be on the same domain as your domain name.
In fact, it might actually be more secure, as free services may have security holes that would allow an attacker to hijack your free account and steal your domain name.