In Islamic law, there is a maxim which states that "Everything is permissible unless it's forbidden explicitly". So for example, when it comes to food, everything is permissible to eat except for some specific things: pork, dead meat, animals slaughtered in the name of other than Allah, etc. Same with clothing; we can wear whatever we want with some exceptions.
One of the proofs for this maxim comes from the Quran:
"It is He who created for you all of that which is on the earth."
Everything has been created for us to use in general, and we have specific prohibitions. Another proof:
"He has only forbidden to you dead animals, blood, the flesh of swine,
and that which has been dedicated to other than Allah " [2:173]
This means he only forbidden the above, which means that everything else is allowed for us to eat (keep in mind there are more exceptions than what is in this verse, but I hope you get the point)
So based on this maxim, when it comes anything in our life which doesn't have a specific prohibition for it, the general rule is that is permissible. Then we look closer at the harms and benefits of that matter.
When it comes to something permissible, you will always find people abusing it. Question: is using the internet allowed? People use and abuse it for various forbidden actions (too many to list!), but that does not change the general ruling that it is permissible. However, for a specific person, it may be impermissible for them specifically if they cannot control themselves from doing impermissible actions and it outweighs the benefits for them.
Same for social networking. It is permissible in general, and people abuse it (flirting, fornication, etc.), while other people use it to call people to Allah and learn Islamic knowledge (which is good)! So it depends on the person and their usage of it.