When you sign up you can connect to like minded individuals who are as passionate as you are about being apart of something that may turn out to be big. You control and own your data. Other big Social Networks owns your digital content and we all know how big Content Marketing is going to be, it is already having an effect on Google's Search Algorithm with the recent panda updates.
As a Internet marketer and a webmaster I can promote a product that I own a part of and not have to worry about keeping up with the maintenance of full ownership on a digital property. Like I said above, you can create content and take ownership of it. You will not have a big wig telling you what privacy policies you have to abide by and how much advertising should be targeted towards you.