Synagogue Web sites are — after an agonizingly slow start — coming of age. Rabbis are blogging and posting sermons on YouTube. Members are signing up and paying for classes online. And several synagogues have launched virtual yahrtzeit boards — complete with e-mail reminders. With the High Holy Days past, many synagogues are contemplating ways to fill their pews on a regular basis. As counter-intuitive as it may seem, the key to building a physical sense of community may well lie in enhancing a synagogue’s online community, social media experts say. That’s why, despite the slumped economy, more and more congregations are realizing the importance of investing in fresh, easy-to-navigate synagogue Web sites (preferably equipped with “donate now” buttons).