Only a reliable stress test can determine how much it will cost roughly to have a certain amount users online.
That being said, i find the infos on the article a little unaccurate since it assumes among 10 k members a slight amount only will be online at the same time. Calculation seems a little strange to me.
Plus the cost of the hosting is high ...very high for a free open source ... i stress tested Phpfox, another social network script, and i could reach 400 online user in a 100-125 dollars a month server structure ...
We need clarification from the UNA team guys ... Thank you.