I can only ever see this as a kind of broken transference of emotions that normally attach to the pleasures (and trials) of proximity to one's friends.
Most simplistically, we have traversed a social arc between almost hard - wired numerical limits to groups, by tribe or family or township, or travelling together, through dissociation of conurbations and suburbia and the popularism of the "atomic family" which mae feel-good about a life within a picket fence and a statutory numberof offspring, all separated from relations, co-workers and even neighbors by white-picket and loosely connecting these nodes via tin chariot (well, steel, but tin chariot scans more to my liking), through synchronous but limited (by cost of area code) node to node networks (POTS) until today one only has to sign up for Facebook and one is inundated not only with "friends" * but the immediacy of multifaceted interaction in many time-division multiplexes of attention, which is in comparison with our earlier means to communicate, infinitely closer to the real thing of mingling among people we know or vaguely know.
Meanwhile, for a litany of reasons, from specialisation in study to the economic drive that demands ever greater input from students to adjuncts and in similar fashion in business demand for narrow "verticals", silos of expertise, the straining naturally not automatically robust interdisciplinary lines of correspondence, even to the distraction of once innocently the snake game, but our phones themselves...
It feels to me as if we have been corralled, and I do not say this has been to any grand plan, but it feels to me as if our lives are inverted. Or at least I felt that I experienced a inversion: My work was a primary social interaction, and home a "refuge" or rather a seclusion at best, a isolation at worst, and it could feel so even when not alone, if I was in my wife's bad books. But now instantaneously I can obtain the adrenaline, serotonin, the chemistry caused by high value, high consequence socialising, whilst individually I cannot communicate silently my mood with a gesture or sigh, whilst individually I can be picked off for especial treatment to pump my brain chemistry in the right way to receive a advertisement, without someone guffawing what a load of tripe the product is or telling me I won't like a violent scene in a movie or..
I may not be doing so well at this, but I am trying to describe how I believe Facebook and imitators can co-opt the physical self that is keyed to complex social interaction, [edit to add next few words] and in a larger social context high value things such a reputation and acceptance carry heavy processing burdens as well, which potentially may impair other critical thought. [end edit]
Along with the chemical and thence psychological rewards, comes very possibly "being happy" or "a happy user", yet because we are alone, a sole observer, I see risks. Even in company we have created our unique context. I recently described the difficulty of discussing code between two on even the same part of a project, to being able to tell from what they say how, by reading what articles or using what sites, your love on the sofa with you has arrived at a particular remark, without them explaining it. I said just keep adding bells on that one." Because social interaction at a wider mark is high value and high risk, and because we process so much of this intuitively or subconsciously, any extension of the framework that omits balances and checks, which are provided in face to face society where communication is necessarily open, I believe there is significant potential for undesirable system wide side effects on a individual.
So I do not think that users are in of themselves happy _because of a product like Facebook, but Facebook has created for them a simulacrum. I believe like any simulation which is necessarily crude, inherently non - deterministic, and isolating whilst increasing sensory or other input, such a system is not inherently not dangerous.
*That experience was made even fresher to me, not very long ago, when I first time ever registered on FB, and before anything else, entered a pre-pay cell number. I was quite confused a while how many people seemed to know me, even invite me to be friends, who seemed to stretch my recollection a bit much, before I twigged the number had been allocated not long before I bought the phone. Invites to be friends were surprising - I checked and I don't share a name with anyone in this other social group...
[Edits for punctuation, minor clarity]