Meet Up is one source for finding others with similar interests, not just hiking, btw. Other sites such as Views from the Top are geographically oriented. VFTT is a NE site with regional trail conditions as well as a general backpacking forum but it's not really a social networking site that links people up like Facebook or LinkedIn.

Keep in mind that the skill levels of people you meet online may not be as advertised. Back in 2007, a Meet Up group of beginner winter campers had to be rescued by a big SAR effort during a major blizzard in the PNW because of poor planning by the group leader.