Is it possible to find an agent for a novel if you don't have a website or social network platform?
Yes, it's absolutely possible. If an agent loves your book enough, he or she will sign you, and the two of you can work together on any platform issues, if necessary, before submitting your manuscript to publishers. That being said, creating a website is relatively easy, and a well-done or particularly creative site can be a plus. Similarly, a larger number of Facebook fans or Twitter followers can tip things in your favor if an agent is on the fence about taking you on as a client.
Being active online, though, is valuable in ways other than just ticking off a marketing box. Online writing communities and social networks can be supportive as well as educational. Twitter, for instance, is a good way to listen in on what agents, book editors, authors, magazine editors, bookstore owners, and others are talking about and can be an efficient way to take the pulse of the industry, which can help you on your path. But remember it's the book that counts, so don't get too distracted.