Yes, clonazepam, brand name Klonopin, can be used in social phobia (social anxiety disorder) and GAD (generalized anxiety disorder). However, since social phobia and GAD are chronic anxiety disorders, it is not recommended to treat these disorders on a long term basis with benzodiazepines such as clonazepam. Certainly, an advantage of benzodiazepines is that they are fast acting for the reduction of anxiety. And clonazepam is advantageous over the other benzodiazepines like lorazepam (Ativan) in that clonazepam is long acting, and dosing it twice daily can cover your anxiety symptoms for the whole day.
The disadvantages of benzodiazepines are that they are highly addictive and have multiple side effects. Side effects include sedation, tremor, dizziness, and drug-drug interactions with other central nervous system (CNS) depressants like alcohol and barbiturates. And when the benzodiazepine dose wears off, then your anxiety usually returns.
So if you want immediate relief from severe anxiety symptoms that are intolerable, then benzodiazepines will most likely work and be fast-acting to relieve your anxiety symptoms. However, as the benzodiazepines have an addictive potential, you might get hooked, and it may be difficult to stop.
Clinically, benzodiazepines are usually prescribed for people with social phobia or GAD who are started on an antidepressant for the treatment of anxiety. The problem with antidepressants, SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin norepinephrine reuptake inhibitors), is that they can take weeks to work for anxiety, and they can also induce activation (agitation and more anxiety) initially. So to counteract this, benzodiazepines, like clonazepam, are prescribed short term, then tapered and discontinued when the SSRI or SNRI becomes effective for anxiety weeks later. It is rare for benzodiazepines to be prescribed as monotherapy (meaning the only medication prescribed) for social phobia or GAD.