When a client approach a designer via social media to make a design job, how to deliver that project to the client keeping a security payment? The designer can't just mail the project and wait for client to pay. What if they just vanish? Same goes for designer side. So is there any convenient way to deal this?
Remote doesn't mean you can't obtain a signed contract. Make sure you get a signature, communicate via email like PP has mentionned and also get other client coordinates (phone number, physical address, etc.) and check if you can find evidence of their existence. Ask for a deposit. At least if the client defaults, you'll have half the payment in hand. Clients who you have physical access to can also vanish or stall a payment.
Use your intuition. Check how the client has found you. Was it through the referral of a good client? If so, it's my experience that good clients tend to attract more good clients but watch for red flags. I've worked with agencies remotely for years without meeting anyone and also have had small businesses consult with me without having ever met me and they were some of my best clients. Don't let distance hold you back from doing business with someone!
There may be some pretty big pros to working remotely depending on where you live and where your client lives. I was in Canada designing for an American business for years while the exchange rate was extremely beneficial for us. It literally increased my hourly rate by 50% for a while. Also, because it was considered an exportation, it was free of tax so that was an advantage for my client.
Payment wise, you can use PayPal though they hold onto some rate for businesses. You can avoid this if you get sent money as a "friend". But depending on where you live, there are other ways like a mailed cheque if you can wait for it to arrive or sending money via SMS through a bank's site.
After finding each other do all legally meaningful communication via single email account and demand the other do the same.
In social media fake profiles have short history and often a stolen images, which can be found from elsewhere.
Everything which needs heavy efforts should be also have a signed agreement and a proven existence via other channels. A client or designer who wants to stay faceless hides something.
Payments should be received via PayPal's business account which is connected to the communication email address. PayPal accepts only persons who have at least some creditability to be business account holders.
No delivery before payment except as heavily watermarked low resolution JPG files. Even then the designer can find that he has given ideas for free.
Final delivery via an established cloud service which provides expiring links, records downloads and gives a possiblity to prove that certain link has been used. Dropbox has one.