Can you name a hobby where one never actually completes anything? Even if a hobby is simply a way to spend time enjoyably, there's usually an end result. For example, a final piece fits into the jigsaw puzzle and one can see the completed picture. We can easily assume reading a novel is completed when the words on the last page end. I suppose even knitting concludes when both pair of mittens are complete.

There is no point at which
a social networking website is completed. Making virtual buddies is one end result, I guess. It's a permanent, ongoing process somewhat akin to having penpals. It's more immediate and gratifying as it's much faster than dealing with international mail. It's cheaper too as long as one's already paying for Net access.
In the end though, the only thing that one can demonstrate is the increasing number of people in the social network. That, assuredly, is directly proportional to the amount of time one devotes to the process.
Lest you think I'm voicing a distaste for social networking, I'm not. It's no more of a waste of time than any other diversion and potentially, it could introduce one to places and people one might have no other way of knowing.